Fixed MTP to work with TWRP

Documentation/virtual/00-INDEX

@@ -0,0 +1,8 @@
+Virtualization support in the Linux kernel.
+
+00-INDEX
+	- this file.
+kvm/
+	- Kernel Virtual Machine.  See also http://linux-kvm.org
+uml/
+	- User Mode Linux, builds/runs Linux kernel as a userspace program.

Documentation/virtual/kvm/00-INDEX

@@ -0,0 +1,26 @@
+00-INDEX
+	- this file.
+api.txt
+	- KVM userspace API.
+cpuid.txt
+	- KVM-specific cpuid leaves (x86).
+devices/
+	- KVM_CAP_DEVICE_CTRL userspace API.
+hypercalls.txt
+	- KVM hypercalls.
+locking.txt
+	- notes on KVM locks.
+mmu.txt
+	- the x86 kvm shadow mmu.
+msr.txt
+	- KVM-specific MSRs (x86).
+nested-vmx.txt
+	- notes on nested virtualization for Intel x86 processors.
+ppc-pv.txt
+	- the paravirtualization interface on PowerPC.
+review-checklist.txt
+	- review checklist for KVM patches.
+s390-diag.txt
+	- Diagnose hypercall description (for IBM S/390)
+timekeeping.txt
+	- timekeeping virtualization for x86-based architectures.

Documentation/virtual/kvm/cpuid.txt

@@ -0,0 +1,60 @@
+KVM CPUID bits
+Glauber Costa <glommer@redhat.com>, Red Hat Inc, 2010
+=====================================================
+
+A guest running on a kvm host, can check some of its features using
+cpuid. This is not always guaranteed to work, since userspace can
+mask-out some, or even all KVM-related cpuid features before launching
+a guest.
+
+KVM cpuid functions are:
+
+function: KVM_CPUID_SIGNATURE (0x40000000)
+returns : eax = 0x40000001,
+          ebx = 0x4b4d564b,
+          ecx = 0x564b4d56,
+          edx = 0x4d.
+Note that this value in ebx, ecx and edx corresponds to the string "KVMKVMKVM".
+The value in eax corresponds to the maximum cpuid function present in this leaf,
+and will be updated if more functions are added in the future.
+Note also that old hosts set eax value to 0x0. This should
+be interpreted as if the value was 0x40000001.
+This function queries the presence of KVM cpuid leafs.
+
+
+function: define KVM_CPUID_FEATURES (0x40000001)
+returns : ebx, ecx, edx = 0
+          eax = and OR'ed group of (1 << flag), where each flags is:
+
+
+flag                               || value || meaning
+=============================================================================
+KVM_FEATURE_CLOCKSOURCE            ||     0 || kvmclock available at msrs
+                                   ||       || 0x11 and 0x12.
+------------------------------------------------------------------------------
+KVM_FEATURE_NOP_IO_DELAY           ||     1 || not necessary to perform delays
+                                   ||       || on PIO operations.
+------------------------------------------------------------------------------
+KVM_FEATURE_MMU_OP                 ||     2 || deprecated.
+------------------------------------------------------------------------------
+KVM_FEATURE_CLOCKSOURCE2           ||     3 || kvmclock available at msrs
+                                   ||       || 0x4b564d00 and 0x4b564d01
+------------------------------------------------------------------------------
+KVM_FEATURE_ASYNC_PF               ||     4 || async pf can be enabled by
+                                   ||       || writing to msr 0x4b564d02
+------------------------------------------------------------------------------
+KVM_FEATURE_STEAL_TIME             ||     5 || steal time can be enabled by
+                                   ||       || writing to msr 0x4b564d03.
+------------------------------------------------------------------------------
+KVM_FEATURE_PV_EOI                 ||     6 || paravirtualized end of interrupt
+                                   ||       || handler can be enabled by writing
+                                   ||       || to msr 0x4b564d04.
+------------------------------------------------------------------------------
+KVM_FEATURE_PV_UNHALT              ||     7 || guest checks this feature bit
+                                   ||       || before enabling paravirtualized
+                                   ||       || spinlock support.
+------------------------------------------------------------------------------
+KVM_FEATURE_CLOCKSOURCE_STABLE_BIT ||    24 || host will warn if no guest-side
+                                   ||       || per-cpu warps are expected in
+                                   ||       || kvmclock.
+------------------------------------------------------------------------------

Documentation/virtual/kvm/devices/README

@@ -0,0 +1 @@
+This directory contains specific device bindings for KVM_CAP_DEVICE_CTRL.

Documentation/virtual/kvm/devices/arm-vgic.txt

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+ARM Virtual Generic Interrupt Controller (VGIC)
+===============================================
+
+Device types supported:
+  KVM_DEV_TYPE_ARM_VGIC_V2     ARM Generic Interrupt Controller v2.0
+
+Only one VGIC instance may be instantiated through either this API or the
+legacy KVM_CREATE_IRQCHIP api.  The created VGIC will act as the VM interrupt
+controller, requiring emulated user-space devices to inject interrupts to the
+VGIC instead of directly to CPUs.
+
+Groups:
+  KVM_DEV_ARM_VGIC_GRP_ADDR
+  Attributes:
+    KVM_VGIC_V2_ADDR_TYPE_DIST (rw, 64-bit)
+      Base address in the guest physical address space of the GIC distributor
+      register mappings.
+
+    KVM_VGIC_V2_ADDR_TYPE_CPU (rw, 64-bit)
+      Base address in the guest physical address space of the GIC virtual cpu
+      interface register mappings.
+
+  KVM_DEV_ARM_VGIC_GRP_DIST_REGS
+  Attributes:
+    The attr field of kvm_device_attr encodes two values:
+    bits:     | 63   ....  40 | 39 ..  32  |  31   ....    0 |
+    values:   |    reserved   |   cpu id   |      offset     |
+
+    All distributor regs are (rw, 32-bit)
+
+    The offset is relative to the "Distributor base address" as defined in the
+    GICv2 specs.  Getting or setting such a register has the same effect as
+    reading or writing the register on the actual hardware from the cpu
+    specified with cpu id field.  Note that most distributor fields are not
+    banked, but return the same value regardless of the cpu id used to access
+    the register.
+  Limitations:
+    - Priorities are not implemented, and registers are RAZ/WI
+  Errors:
+    -ENODEV: Getting or setting this register is not yet supported
+    -EBUSY: One or more VCPUs are running
+
+  KVM_DEV_ARM_VGIC_GRP_CPU_REGS
+  Attributes:
+    The attr field of kvm_device_attr encodes two values:
+    bits:     | 63   ....  40 | 39 ..  32  |  31   ....    0 |
+    values:   |    reserved   |   cpu id   |      offset     |
+
+    All CPU interface regs are (rw, 32-bit)
+
+    The offset specifies the offset from the "CPU interface base address" as
+    defined in the GICv2 specs.  Getting or setting such a register has the
+    same effect as reading or writing the register on the actual hardware.
+
+    The Active Priorities Registers APRn are implementation defined, so we set a
+    fixed format for our implementation that fits with the model of a "GICv2
+    implementation without the security extensions" which we present to the
+    guest.  This interface always exposes four register APR[0-3] describing the
+    maximum possible 128 preemption levels.  The semantics of the register
+    indicate if any interrupts in a given preemption level are in the active
+    state by setting the corresponding bit.
+
+    Thus, preemption level X has one or more active interrupts if and only if:
+
+      APRn[X mod 32] == 0b1,  where n = X / 32
+
+    Bits for undefined preemption levels are RAZ/WI.
+
+  Limitations:
+    - Priorities are not implemented, and registers are RAZ/WI
+  Errors:
+    -ENODEV: Getting or setting this register is not yet supported
+    -EBUSY: One or more VCPUs are running
+
+  KVM_DEV_ARM_VGIC_GRP_NR_IRQS
+  Attributes:
+    A value describing the number of interrupts (SGI, PPI and SPI) for
+    this GIC instance, ranging from 64 to 1024, in increments of 32.
+
+  Errors:
+    -EINVAL: Value set is out of the expected range
+    -EBUSY: Value has already be set, or GIC has already been initialized
+            with default values.

Documentation/virtual/kvm/devices/mpic.txt

@@ -0,0 +1,53 @@
+MPIC interrupt controller
+=========================
+
+Device types supported:
+  KVM_DEV_TYPE_FSL_MPIC_20     Freescale MPIC v2.0
+  KVM_DEV_TYPE_FSL_MPIC_42     Freescale MPIC v4.2
+
+Only one MPIC instance, of any type, may be instantiated.  The created
+MPIC will act as the system interrupt controller, connecting to each
+vcpu's interrupt inputs.
+
+Groups:
+  KVM_DEV_MPIC_GRP_MISC
+  Attributes:
+    KVM_DEV_MPIC_BASE_ADDR (rw, 64-bit)
+      Base address of the 256 KiB MPIC register space.  Must be
+      naturally aligned.  A value of zero disables the mapping.
+      Reset value is zero.
+
+  KVM_DEV_MPIC_GRP_REGISTER (rw, 32-bit)
+    Access an MPIC register, as if the access were made from the guest.
+    "attr" is the byte offset into the MPIC register space.  Accesses
+    must be 4-byte aligned.
+
+    MSIs may be signaled by using this attribute group to write
+    to the relevant MSIIR.
+
+  KVM_DEV_MPIC_GRP_IRQ_ACTIVE (rw, 32-bit)
+    IRQ input line for each standard openpic source.  0 is inactive and 1
+    is active, regardless of interrupt sense.
+
+    For edge-triggered interrupts:  Writing 1 is considered an activating
+    edge, and writing 0 is ignored.  Reading returns 1 if a previously
+    signaled edge has not been acknowledged, and 0 otherwise.
+
+    "attr" is the IRQ number.  IRQ numbers for standard sources are the
+    byte offset of the relevant IVPR from EIVPR0, divided by 32.
+
+IRQ Routing:
+
+  The MPIC emulation supports IRQ routing. Only a single MPIC device can
+  be instantiated. Once that device has been created, it's available as
+  irqchip id 0.
+
+  This irqchip 0 has 256 interrupt pins, which expose the interrupts in
+  the main array of interrupt sources (a.k.a. "SRC" interrupts).
+
+  The numbering is the same as the MPIC device tree binding -- based on
+  the register offset from the beginning of the sources array, without
+  regard to any subdivisions in chip documentation such as "internal"
+  or "external" interrupts.
+
+  Access to non-SRC interrupts is not implemented through IRQ routing mechanisms.

Documentation/virtual/kvm/devices/s390_flic.txt

@@ -0,0 +1,94 @@
+FLIC (floating interrupt controller)
+====================================
+
+FLIC handles floating (non per-cpu) interrupts, i.e. I/O, service and some
+machine check interruptions. All interrupts are stored in a per-vm list of
+pending interrupts. FLIC performs operations on this list.
+
+Only one FLIC instance may be instantiated.
+
+FLIC provides support to
+- add interrupts (KVM_DEV_FLIC_ENQUEUE)
+- inspect currently pending interrupts (KVM_FLIC_GET_ALL_IRQS)
+- purge all pending floating interrupts (KVM_DEV_FLIC_CLEAR_IRQS)
+- enable/disable for the guest transparent async page faults
+- register and modify adapter interrupt sources (KVM_DEV_FLIC_ADAPTER_*)
+
+Groups:
+  KVM_DEV_FLIC_ENQUEUE
+    Passes a buffer and length into the kernel which are then injected into
+    the list of pending interrupts.
+    attr->addr contains the pointer to the buffer and attr->attr contains
+    the length of the buffer.
+    The format of the data structure kvm_s390_irq as it is copied from userspace
+    is defined in usr/include/linux/kvm.h.
+
+  KVM_DEV_FLIC_GET_ALL_IRQS
+    Copies all floating interrupts into a buffer provided by userspace.
+    When the buffer is too small it returns -ENOMEM, which is the indication
+    for userspace to try again with a bigger buffer.
+    -ENOBUFS is returned when the allocation of a kernelspace buffer has
+    failed.
+    -EFAULT is returned when copying data to userspace failed.
+    All interrupts remain pending, i.e. are not deleted from the list of
+    currently pending interrupts.
+    attr->addr contains the userspace address of the buffer into which all
+    interrupt data will be copied.
+    attr->attr contains the size of the buffer in bytes.
+
+  KVM_DEV_FLIC_CLEAR_IRQS
+    Simply deletes all elements from the list of currently pending floating
+    interrupts.  No interrupts are injected into the guest.
+
+  KVM_DEV_FLIC_APF_ENABLE
+    Enables async page faults for the guest. So in case of a major page fault
+    the host is allowed to handle this async and continues the guest.
+
+  KVM_DEV_FLIC_APF_DISABLE_WAIT
+    Disables async page faults for the guest and waits until already pending
+    async page faults are done. This is necessary to trigger a completion interrupt
+    for every init interrupt before migrating the interrupt list.
+
+  KVM_DEV_FLIC_ADAPTER_REGISTER
+    Register an I/O adapter interrupt source. Takes a kvm_s390_io_adapter
+    describing the adapter to register:
+
+struct kvm_s390_io_adapter {
+	__u32 id;
+	__u8 isc;
+	__u8 maskable;
+	__u8 swap;
+	__u8 pad;
+};
+
+   id contains the unique id for the adapter, isc the I/O interruption subclass
+   to use, maskable whether this adapter may be masked (interrupts turned off)
+   and swap whether the indicators need to be byte swapped.
+
+
+  KVM_DEV_FLIC_ADAPTER_MODIFY
+    Modifies attributes of an existing I/O adapter interrupt source. Takes
+    a kvm_s390_io_adapter_req specifiying the adapter and the operation:
+
+struct kvm_s390_io_adapter_req {
+	__u32 id;
+	__u8 type;
+	__u8 mask;
+	__u16 pad0;
+	__u64 addr;
+};
+
+    id specifies the adapter and type the operation. The supported operations
+    are:
+
+    KVM_S390_IO_ADAPTER_MASK
+      mask or unmask the adapter, as specified in mask
+
+    KVM_S390_IO_ADAPTER_MAP
+      perform a gmap translation for the guest address provided in addr,
+      pin a userspace page for the translated address and add it to the
+      list of mappings
+
+    KVM_S390_IO_ADAPTER_UNMAP
+      release a userspace page for the translated address specified in addr
+      from the list of mappings

Documentation/virtual/kvm/devices/vfio.txt

@@ -0,0 +1,22 @@
+VFIO virtual device
+===================
+
+Device types supported:
+  KVM_DEV_TYPE_VFIO
+
+Only one VFIO instance may be created per VM.  The created device
+tracks VFIO groups in use by the VM and features of those groups
+important to the correctness and acceleration of the VM.  As groups
+are enabled and disabled for use by the VM, KVM should be updated
+about their presence.  When registered with KVM, a reference to the
+VFIO-group is held by KVM.
+
+Groups:
+  KVM_DEV_VFIO_GROUP
+
+KVM_DEV_VFIO_GROUP attributes:
+  KVM_DEV_VFIO_GROUP_ADD: Add a VFIO group to VFIO-KVM device tracking
+  KVM_DEV_VFIO_GROUP_DEL: Remove a VFIO group from VFIO-KVM device tracking
+
+For each, kvm_device_attr.addr points to an int32_t file descriptor
+for the VFIO group.

Documentation/virtual/kvm/devices/vm.txt

@@ -0,0 +1,26 @@
+Generic vm interface
+====================================
+
+The virtual machine "device" also accepts the ioctls KVM_SET_DEVICE_ATTR,
+KVM_GET_DEVICE_ATTR, and KVM_HAS_DEVICE_ATTR. The interface uses the same
+struct kvm_device_attr as other devices, but targets VM-wide settings
+and controls.
+
+The groups and attributes per virtual machine, if any, are architecture
+specific.
+
+1. GROUP: KVM_S390_VM_MEM_CTRL
+Architectures: s390
+
+1.1. ATTRIBUTE: KVM_S390_VM_MEM_CTRL
+Parameters: none
+Returns: -EBUSY if already a vcpus is defined, otherwise 0
+
+Enables CMMA for the virtual machine
+
+1.2. ATTRIBUTE: KVM_S390_VM_CLR_CMMA
+Parameteres: none
+Returns: 0
+
+Clear the CMMA status for all guest pages, so any pages the guest marked
+as unused are again used any may not be reclaimed by the host.

Documentation/virtual/kvm/devices/xics.txt

@@ -0,0 +1,66 @@
+XICS interrupt controller
+
+Device type supported: KVM_DEV_TYPE_XICS
+
+Groups:
+  KVM_DEV_XICS_SOURCES
+  Attributes: One per interrupt source, indexed by the source number.
+
+This device emulates the XICS (eXternal Interrupt Controller
+Specification) defined in PAPR.  The XICS has a set of interrupt
+sources, each identified by a 20-bit source number, and a set of
+Interrupt Control Presentation (ICP) entities, also called "servers",
+each associated with a virtual CPU.
+
+The ICP entities are created by enabling the KVM_CAP_IRQ_ARCH
+capability for each vcpu, specifying KVM_CAP_IRQ_XICS in args[0] and
+the interrupt server number (i.e. the vcpu number from the XICS's
+point of view) in args[1] of the kvm_enable_cap struct.  Each ICP has
+64 bits of state which can be read and written using the
+KVM_GET_ONE_REG and KVM_SET_ONE_REG ioctls on the vcpu.  The 64 bit
+state word has the following bitfields, starting at the
+least-significant end of the word:
+
+* Unused, 16 bits
+
+* Pending interrupt priority, 8 bits
+  Zero is the highest priority, 255 means no interrupt is pending.
+
+* Pending IPI (inter-processor interrupt) priority, 8 bits
+  Zero is the highest priority, 255 means no IPI is pending.
+
+* Pending interrupt source number, 24 bits
+  Zero means no interrupt pending, 2 means an IPI is pending
+
+* Current processor priority, 8 bits
+  Zero is the highest priority, meaning no interrupts can be
+  delivered, and 255 is the lowest priority.
+
+Each source has 64 bits of state that can be read and written using
+the KVM_GET_DEVICE_ATTR and KVM_SET_DEVICE_ATTR ioctls, specifying the
+KVM_DEV_XICS_SOURCES attribute group, with the attribute number being
+the interrupt source number.  The 64 bit state word has the following
+bitfields, starting from the least-significant end of the word:
+
+* Destination (server number), 32 bits
+  This specifies where the interrupt should be sent, and is the
+  interrupt server number specified for the destination vcpu.
+
+* Priority, 8 bits
+  This is the priority specified for this interrupt source, where 0 is
+  the highest priority and 255 is the lowest.  An interrupt with a
+  priority of 255 will never be delivered.
+
+* Level sensitive flag, 1 bit
+  This bit is 1 for a level-sensitive interrupt source, or 0 for
+  edge-sensitive (or MSI).
+
+* Masked flag, 1 bit
+  This bit is set to 1 if the interrupt is masked (cannot be delivered
+  regardless of its priority), for example by the ibm,int-off RTAS
+  call, or 0 if it is not masked.
+
+* Pending flag, 1 bit
+  This bit is 1 if the source has a pending interrupt, otherwise 0.
+
+Only one XICS instance may be created per VM.

Documentation/virtual/kvm/hypercalls.txt

@@ -0,0 +1,83 @@
+Linux KVM Hypercall:
+===================
+X86:
+ KVM Hypercalls have a three-byte sequence of either the vmcall or the vmmcall
+ instruction. The hypervisor can replace it with instructions that are
+ guaranteed to be supported.
+
+ Up to four arguments may be passed in rbx, rcx, rdx, and rsi respectively.
+ The hypercall number should be placed in rax and the return value will be
+ placed in rax.  No other registers will be clobbered unless explicitly stated
+ by the particular hypercall.
+
+S390:
+  R2-R7 are used for parameters 1-6. In addition, R1 is used for hypercall
+  number. The return value is written to R2.
+
+  S390 uses diagnose instruction as hypercall (0x500) along with hypercall
+  number in R1.
+
+  For further information on the S390 diagnose call as supported by KVM,
+  refer to Documentation/virtual/kvm/s390-diag.txt.
+
+ PowerPC:
+  It uses R3-R10 and hypercall number in R11. R4-R11 are used as output registers.
+  Return value is placed in R3.
+
+  KVM hypercalls uses 4 byte opcode, that are patched with 'hypercall-instructions'
+  property inside the device tree's /hypervisor node.
+  For more information refer to Documentation/virtual/kvm/ppc-pv.txt
+
+KVM Hypercalls Documentation
+===========================
+The template for each hypercall is:
+1. Hypercall name.
+2. Architecture(s)
+3. Status (deprecated, obsolete, active)
+4. Purpose
+
+1. KVM_HC_VAPIC_POLL_IRQ
+------------------------
+Architecture: x86
+Status: active
+Purpose: Trigger guest exit so that the host can check for pending
+interrupts on reentry.
+
+2. KVM_HC_MMU_OP
+------------------------
+Architecture: x86
+Status: deprecated.
+Purpose: Support MMU operations such as writing to PTE,
+flushing TLB, release PT.
+
+3. KVM_HC_FEATURES
+------------------------
+Architecture: PPC
+Status: active
+Purpose: Expose hypercall availability to the guest. On x86 platforms, cpuid
+used to enumerate which hypercalls are available. On PPC, either device tree
+based lookup ( which is also what EPAPR dictates) OR KVM specific enumeration
+mechanism (which is this hypercall) can be used.
+
+4. KVM_HC_PPC_MAP_MAGIC_PAGE
+------------------------
+Architecture: PPC
+Status: active
+Purpose: To enable communication between the hypervisor and guest there is a
+shared page that contains parts of supervisor visible register state.
+The guest can map this shared page to access its supervisor register through
+memory using this hypercall.
+
+5. KVM_HC_KICK_CPU
+------------------------
+Architecture: x86
+Status: active
+Purpose: Hypercall used to wakeup a vcpu from HLT state
+Usage example : A vcpu of a paravirtualized guest that is busywaiting in guest
+kernel mode for an event to occur (ex: a spinlock to become available) can
+execute HLT instruction once it has busy-waited for more than a threshold
+time-interval. Execution of HLT instruction would cause the hypervisor to put
+the vcpu to sleep until occurrence of an appropriate event. Another vcpu of the
+same guest can wakeup the sleeping vcpu by issuing KVM_HC_KICK_CPU hypercall,
+specifying APIC ID (a1) of the vcpu to be woken up. An additional argument (a0)
+is used in the hypercall for future use.

Documentation/virtual/kvm/locking.txt

@@ -0,0 +1,168 @@
+KVM Lock Overview
+=================
+
+1. Acquisition Orders
+---------------------
+
+(to be written)
+
+2: Exception
+------------
+
+Fast page fault:
+
+Fast page fault is the fast path which fixes the guest page fault out of
+the mmu-lock on x86. Currently, the page fault can be fast only if the
+shadow page table is present and it is caused by write-protect, that means
+we just need change the W bit of the spte.
+
+What we use to avoid all the race is the SPTE_HOST_WRITEABLE bit and
+SPTE_MMU_WRITEABLE bit on the spte:
+- SPTE_HOST_WRITEABLE means the gfn is writable on host.
+- SPTE_MMU_WRITEABLE means the gfn is writable on mmu. The bit is set when
+  the gfn is writable on guest mmu and it is not write-protected by shadow
+  page write-protection.
+
+On fast page fault path, we will use cmpxchg to atomically set the spte W
+bit if spte.SPTE_HOST_WRITEABLE = 1 and spte.SPTE_WRITE_PROTECT = 1, this
+is safe because whenever changing these bits can be detected by cmpxchg.
+
+But we need carefully check these cases:
+1): The mapping from gfn to pfn
+The mapping from gfn to pfn may be changed since we can only ensure the pfn
+is not changed during cmpxchg. This is a ABA problem, for example, below case
+will happen:
+
+At the beginning:
+gpte = gfn1
+gfn1 is mapped to pfn1 on host
+spte is the shadow page table entry corresponding with gpte and
+spte = pfn1
+
+   VCPU 0                           VCPU0
+on fast page fault path:
+
+   old_spte = *spte;
+                                 pfn1 is swapped out:
+                                    spte = 0;
+
+                                 pfn1 is re-alloced for gfn2.
+
+                                 gpte is changed to point to
+                                 gfn2 by the guest:
+                                    spte = pfn1;
+
+   if (cmpxchg(spte, old_spte, old_spte+W)
+	mark_page_dirty(vcpu->kvm, gfn1)
+             OOPS!!!
+
+We dirty-log for gfn1, that means gfn2 is lost in dirty-bitmap.
+
+For direct sp, we can easily avoid it since the spte of direct sp is fixed
+to gfn. For indirect sp, before we do cmpxchg, we call gfn_to_pfn_atomic()
+to pin gfn to pfn, because after gfn_to_pfn_atomic():
+- We have held the refcount of pfn that means the pfn can not be freed and
+  be reused for another gfn.
+- The pfn is writable that means it can not be shared between different gfns
+  by KSM.
+
+Then, we can ensure the dirty bitmaps is correctly set for a gfn.
+
+Currently, to simplify the whole things, we disable fast page fault for
+indirect shadow page.
+
+2): Dirty bit tracking
+In the origin code, the spte can be fast updated (non-atomically) if the
+spte is read-only and the Accessed bit has already been set since the
+Accessed bit and Dirty bit can not be lost.
+
+But it is not true after fast page fault since the spte can be marked
+writable between reading spte and updating spte. Like below case:
+
+At the beginning:
+spte.W = 0
+spte.Accessed = 1
+
+   VCPU 0                                       VCPU0
+In mmu_spte_clear_track_bits():
+
+   old_spte = *spte;
+
+   /* 'if' condition is satisfied. */
+   if (old_spte.Accssed == 1 &&
+        old_spte.W == 0)
+      spte = 0ull;
+                                         on fast page fault path:
+                                             spte.W = 1
+                                         memory write on the spte:
+                                             spte.Dirty = 1
+
+
+   else
+      old_spte = xchg(spte, 0ull)
+
+
+   if (old_spte.Accssed == 1)
+      kvm_set_pfn_accessed(spte.pfn);
+   if (old_spte.Dirty == 1)
+      kvm_set_pfn_dirty(spte.pfn);
+      OOPS!!!
+
+The Dirty bit is lost in this case.
+
+In order to avoid this kind of issue, we always treat the spte as "volatile"
+if it can be updated out of mmu-lock, see spte_has_volatile_bits(), it means,
+the spte is always atomically updated in this case.
+
+3): flush tlbs due to spte updated
+If the spte is updated from writable to readonly, we should flush all TLBs,
+otherwise rmap_write_protect will find a read-only spte, even though the
+writable spte might be cached on a CPU's TLB.
+
+As mentioned before, the spte can be updated to writable out of mmu-lock on
+fast page fault path, in order to easily audit the path, we see if TLBs need
+be flushed caused by this reason in mmu_spte_update() since this is a common
+function to update spte (present -> present).
+
+Since the spte is "volatile" if it can be updated out of mmu-lock, we always
+atomically update the spte, the race caused by fast page fault can be avoided,
+See the comments in spte_has_volatile_bits() and mmu_spte_update().
+
+3. Reference
+------------
+
+Name:		kvm_lock
+Type:		spinlock_t
+Arch:		any
+Protects:	- vm_list
+
+Name:		kvm_count_lock
+Type:		raw_spinlock_t
+Arch:		any
+Protects:	- hardware virtualization enable/disable
+Comment:	'raw' because hardware enabling/disabling must be atomic /wrt
+		migration.
+
+Name:		kvm_arch::tsc_write_lock
+Type:		raw_spinlock
+Arch:		x86
+Protects:	- kvm_arch::{last_tsc_write,last_tsc_nsec,last_tsc_offset}
+		- tsc offset in vmcb
+Comment:	'raw' because updating the tsc offsets must not be preempted.
+
+Name:		kvm->mmu_lock
+Type:		spinlock_t
+Arch:		any
+Protects:	-shadow page/shadow tlb entry
+Comment:	it is a spinlock since it is used in mmu notifier.
+
+Name:		kvm->srcu
+Type:		srcu lock
+Arch:		any
+Protects:	- kvm->memslots
+		- kvm->buses
+Comment:	The srcu read lock must be held while accessing memslots (e.g.
+		when using gfn_to_* functions) and while accessing in-kernel
+		MMIO/PIO address->device structure mapping (kvm->buses).
+		The srcu index can be stored in kvm_vcpu->srcu_idx per vcpu
+		if it is needed by multiple functions.

Documentation/virtual/kvm/mmu.txt

@@ -0,0 +1,448 @@
+The x86 kvm shadow mmu
+======================
+
+The mmu (in arch/x86/kvm, files mmu.[ch] and paging_tmpl.h) is responsible
+for presenting a standard x86 mmu to the guest, while translating guest
+physical addresses to host physical addresses.
+
+The mmu code attempts to satisfy the following requirements:
+
+- correctness: the guest should not be able to determine that it is running
+               on an emulated mmu except for timing (we attempt to comply
+               with the specification, not emulate the characteristics of
+               a particular implementation such as tlb size)
+- security:    the guest must not be able to touch host memory not assigned
+               to it
+- performance: minimize the performance penalty imposed by the mmu
+- scaling:     need to scale to large memory and large vcpu guests
+- hardware:    support the full range of x86 virtualization hardware
+- integration: Linux memory management code must be in control of guest memory
+               so that swapping, page migration, page merging, transparent
+               hugepages, and similar features work without change
+- dirty tracking: report writes to guest memory to enable live migration
+               and framebuffer-based displays
+- footprint:   keep the amount of pinned kernel memory low (most memory
+               should be shrinkable)
+- reliability:  avoid multipage or GFP_ATOMIC allocations
+
+Acronyms
+========
+
+pfn   host page frame number
+hpa   host physical address
+hva   host virtual address
+gfn   guest frame number
+gpa   guest physical address
+gva   guest virtual address
+ngpa  nested guest physical address
+ngva  nested guest virtual address
+pte   page table entry (used also to refer generically to paging structure
+      entries)
+gpte  guest pte (referring to gfns)
+spte  shadow pte (referring to pfns)
+tdp   two dimensional paging (vendor neutral term for NPT and EPT)
+
+Virtual and real hardware supported
+===================================
+
+The mmu supports first-generation mmu hardware, which allows an atomic switch
+of the current paging mode and cr3 during guest entry, as well as
+two-dimensional paging (AMD's NPT and Intel's EPT).  The emulated hardware
+it exposes is the traditional 2/3/4 level x86 mmu, with support for global
+pages, pae, pse, pse36, cr0.wp, and 1GB pages.  Work is in progress to support
+exposing NPT capable hardware on NPT capable hosts.
+
+Translation
+===========
+
+The primary job of the mmu is to program the processor's mmu to translate
+addresses for the guest.  Different translations are required at different
+times:
+
+- when guest paging is disabled, we translate guest physical addresses to
+  host physical addresses (gpa->hpa)
+- when guest paging is enabled, we translate guest virtual addresses, to
+  guest physical addresses, to host physical addresses (gva->gpa->hpa)
+- when the guest launches a guest of its own, we translate nested guest
+  virtual addresses, to nested guest physical addresses, to guest physical
+  addresses, to host physical addresses (ngva->ngpa->gpa->hpa)
+
+The primary challenge is to encode between 1 and 3 translations into hardware
+that support only 1 (traditional) and 2 (tdp) translations.  When the
+number of required translations matches the hardware, the mmu operates in
+direct mode; otherwise it operates in shadow mode (see below).
+
+Memory
+======
+
+Guest memory (gpa) is part of the user address space of the process that is
+using kvm.  Userspace defines the translation between guest addresses and user
+addresses (gpa->hva); note that two gpas may alias to the same hva, but not
+vice versa.
+
+These hvas may be backed using any method available to the host: anonymous
+memory, file backed memory, and device memory.  Memory might be paged by the
+host at any time.
+
+Events
+======
+
+The mmu is driven by events, some from the guest, some from the host.
+
+Guest generated events:
+- writes to control registers (especially cr3)
+- invlpg/invlpga instruction execution
+- access to missing or protected translations
+
+Host generated events:
+- changes in the gpa->hpa translation (either through gpa->hva changes or
+  through hva->hpa changes)
+- memory pressure (the shrinker)
+
+Shadow pages
+============
+
+The principal data structure is the shadow page, 'struct kvm_mmu_page'.  A
+shadow page contains 512 sptes, which can be either leaf or nonleaf sptes.  A
+shadow page may contain a mix of leaf and nonleaf sptes.
+
+A nonleaf spte allows the hardware mmu to reach the leaf pages and
+is not related to a translation directly.  It points to other shadow pages.
+
+A leaf spte corresponds to either one or two translations encoded into
+one paging structure entry.  These are always the lowest level of the
+translation stack, with optional higher level translations left to NPT/EPT.
+Leaf ptes point at guest pages.
+
+The following table shows translations encoded by leaf ptes, with higher-level
+translations in parentheses:
+
+ Non-nested guests:
+  nonpaging:     gpa->hpa
+  paging:        gva->gpa->hpa
+  paging, tdp:   (gva->)gpa->hpa
+ Nested guests:
+  non-tdp:       ngva->gpa->hpa  (*)
+  tdp:           (ngva->)ngpa->gpa->hpa
+
+(*) the guest hypervisor will encode the ngva->gpa translation into its page
+    tables if npt is not present
+
+Shadow pages contain the following information:
+  role.level:
+    The level in the shadow paging hierarchy that this shadow page belongs to.
+    1=4k sptes, 2=2M sptes, 3=1G sptes, etc.
+  role.direct:
+    If set, leaf sptes reachable from this page are for a linear range.
+    Examples include real mode translation, large guest pages backed by small
+    host pages, and gpa->hpa translations when NPT or EPT is active.
+    The linear range starts at (gfn << PAGE_SHIFT) and its size is determined
+    by role.level (2MB for first level, 1GB for second level, 0.5TB for third
+    level, 256TB for fourth level)
+    If clear, this page corresponds to a guest page table denoted by the gfn
+    field.
+  role.quadrant:
+    When role.cr4_pae=0, the guest uses 32-bit gptes while the host uses 64-bit
+    sptes.  That means a guest page table contains more ptes than the host,
+    so multiple shadow pages are needed to shadow one guest page.
+    For first-level shadow pages, role.quadrant can be 0 or 1 and denotes the
+    first or second 512-gpte block in the guest page table.  For second-level
+    page tables, each 32-bit gpte is converted to two 64-bit sptes
+    (since each first-level guest page is shadowed by two first-level
+    shadow pages) so role.quadrant takes values in the range 0..3.  Each
+    quadrant maps 1GB virtual address space.
+  role.access:
+    Inherited guest access permissions in the form uwx.  Note execute
+    permission is positive, not negative.
+  role.invalid:
+    The page is invalid and should not be used.  It is a root page that is
+    currently pinned (by a cpu hardware register pointing to it); once it is
+    unpinned it will be destroyed.
+  role.cr4_pae:
+    Contains the value of cr4.pae for which the page is valid (e.g. whether
+    32-bit or 64-bit gptes are in use).
+  role.nxe:
+    Contains the value of efer.nxe for which the page is valid.
+  role.cr0_wp:
+    Contains the value of cr0.wp for which the page is valid.
+  role.smep_andnot_wp:
+    Contains the value of cr4.smep && !cr0.wp for which the page is valid
+    (pages for which this is true are different from other pages; see the
+    treatment of cr0.wp=0 below).
+  gfn:
+    Either the guest page table containing the translations shadowed by this
+    page, or the base page frame for linear translations.  See role.direct.
+  spt:
+    A pageful of 64-bit sptes containing the translations for this page.
+    Accessed by both kvm and hardware.
+    The page pointed to by spt will have its page->private pointing back
+    at the shadow page structure.
+    sptes in spt point either at guest pages, or at lower-level shadow pages.
+    Specifically, if sp1 and sp2 are shadow pages, then sp1->spt[n] may point
+    at __pa(sp2->spt).  sp2 will point back at sp1 through parent_pte.
+    The spt array forms a DAG structure with the shadow page as a node, and
+    guest pages as leaves.
+  gfns:
+    An array of 512 guest frame numbers, one for each present pte.  Used to
+    perform a reverse map from a pte to a gfn. When role.direct is set, any
+    element of this array can be calculated from the gfn field when used, in
+    this case, the array of gfns is not allocated. See role.direct and gfn.
+  root_count:
+    A counter keeping track of how many hardware registers (guest cr3 or
+    pdptrs) are now pointing at the page.  While this counter is nonzero, the
+    page cannot be destroyed.  See role.invalid.
+  parent_ptes:
+    The reverse mapping for the pte/ptes pointing at this page's spt. If
+    parent_ptes bit 0 is zero, only one spte points at this pages and
+    parent_ptes points at this single spte, otherwise, there exists multiple
+    sptes pointing at this page and (parent_ptes & ~0x1) points at a data
+    structure with a list of parent_ptes.
+  unsync:
+    If true, then the translations in this page may not match the guest's
+    translation.  This is equivalent to the state of the tlb when a pte is
+    changed but before the tlb entry is flushed.  Accordingly, unsync ptes
+    are synchronized when the guest executes invlpg or flushes its tlb by
+    other means.  Valid for leaf pages.
+  unsync_children:
+    How many sptes in the page point at pages that are unsync (or have
+    unsynchronized children).
+  unsync_child_bitmap:
+    A bitmap indicating which sptes in spt point (directly or indirectly) at
+    pages that may be unsynchronized.  Used to quickly locate all unsychronized
+    pages reachable from a given page.
+  mmu_valid_gen:
+    Generation number of the page.  It is compared with kvm->arch.mmu_valid_gen
+    during hash table lookup, and used to skip invalidated shadow pages (see
+    "Zapping all pages" below.)
+  clear_spte_count:
+    Only present on 32-bit hosts, where a 64-bit spte cannot be written
+    atomically.  The reader uses this while running out of the MMU lock
+    to detect in-progress updates and retry them until the writer has
+    finished the write.
+  write_flooding_count:
+    A guest may write to a page table many times, causing a lot of
+    emulations if the page needs to be write-protected (see "Synchronized
+    and unsynchronized pages" below).  Leaf pages can be unsynchronized
+    so that they do not trigger frequent emulation, but this is not
+    possible for non-leafs.  This field counts the number of emulations
+    since the last time the page table was actually used; if emulation
+    is triggered too frequently on this page, KVM will unmap the page
+    to avoid emulation in the future.
+
+Reverse map
+===========
+
+The mmu maintains a reverse mapping whereby all ptes mapping a page can be
+reached given its gfn.  This is used, for example, when swapping out a page.
+
+Synchronized and unsynchronized pages
+=====================================
+
+The guest uses two events to synchronize its tlb and page tables: tlb flushes
+and page invalidations (invlpg).
+
+A tlb flush means that we need to synchronize all sptes reachable from the
+guest's cr3.  This is expensive, so we keep all guest page tables write
+protected, and synchronize sptes to gptes when a gpte is written.
+
+A special case is when a guest page table is reachable from the current
+guest cr3.  In this case, the guest is obliged to issue an invlpg instruction
+before using the translation.  We take advantage of that by removing write
+protection from the guest page, and allowing the guest to modify it freely.
+We synchronize modified gptes when the guest invokes invlpg.  This reduces
+the amount of emulation we have to do when the guest modifies multiple gptes,
+or when the a guest page is no longer used as a page table and is used for
+random guest data.
+
+As a side effect we have to resynchronize all reachable unsynchronized shadow
+pages on a tlb flush.
+
+
+Reaction to events
+==================
+
+- guest page fault (or npt page fault, or ept violation)
+
+This is the most complicated event.  The cause of a page fault can be:
+
+  - a true guest fault (the guest translation won't allow the access) (*)
+  - access to a missing translation
+  - access to a protected translation
+    - when logging dirty pages, memory is write protected
+    - synchronized shadow pages are write protected (*)
+  - access to untranslatable memory (mmio)
+
+  (*) not applicable in direct mode
+
+Handling a page fault is performed as follows:
+
+ - if the RSV bit of the error code is set, the page fault is caused by guest
+   accessing MMIO and cached MMIO information is available.
+   - walk shadow page table
+   - check for valid generation number in the spte (see "Fast invalidation of
+     MMIO sptes" below)
+   - cache the information to vcpu->arch.mmio_gva, vcpu->arch.access and
+     vcpu->arch.mmio_gfn, and call the emulator
+ - If both P bit and R/W bit of error code are set, this could possibly
+   be handled as a "fast page fault" (fixed without taking the MMU lock).  See
+   the description in Documentation/virtual/kvm/locking.txt.
+ - if needed, walk the guest page tables to determine the guest translation
+   (gva->gpa or ngpa->gpa)
+   - if permissions are insufficient, reflect the fault back to the guest
+ - determine the host page
+   - if this is an mmio request, there is no host page; cache the info to
+     vcpu->arch.mmio_gva, vcpu->arch.access and vcpu->arch.mmio_gfn
+ - walk the shadow page table to find the spte for the translation,
+   instantiating missing intermediate page tables as necessary
+   - If this is an mmio request, cache the mmio info to the spte and set some
+     reserved bit on the spte (see callers of kvm_mmu_set_mmio_spte_mask)
+ - try to unsynchronize the page
+   - if successful, we can let the guest continue and modify the gpte
+ - emulate the instruction
+   - if failed, unshadow the page and let the guest continue
+ - update any translations that were modified by the instruction
+
+invlpg handling:
+
+  - walk the shadow page hierarchy and drop affected translations
+  - try to reinstantiate the indicated translation in the hope that the
+    guest will use it in the near future
+
+Guest control register updates:
+
+- mov to cr3
+  - look up new shadow roots
+  - synchronize newly reachable shadow pages
+
+- mov to cr0/cr4/efer
+  - set up mmu context for new paging mode
+  - look up new shadow roots
+  - synchronize newly reachable shadow pages
+
+Host translation updates:
+
+  - mmu notifier called with updated hva
+  - look up affected sptes through reverse map
+  - drop (or update) translations
+
+Emulating cr0.wp
+================
+
+If tdp is not enabled, the host must keep cr0.wp=1 so page write protection
+works for the guest kernel, not guest guest userspace.  When the guest
+cr0.wp=1, this does not present a problem.  However when the guest cr0.wp=0,
+we cannot map the permissions for gpte.u=1, gpte.w=0 to any spte (the
+semantics require allowing any guest kernel access plus user read access).
+
+We handle this by mapping the permissions to two possible sptes, depending
+on fault type:
+
+- kernel write fault: spte.u=0, spte.w=1 (allows full kernel access,
+  disallows user access)
+- read fault: spte.u=1, spte.w=0 (allows full read access, disallows kernel
+  write access)
+
+(user write faults generate a #PF)
+
+In the first case there is an additional complication if CR4.SMEP is
+enabled: since we've turned the page into a kernel page, the kernel may now
+execute it.  We handle this by also setting spte.nx.  If we get a user
+fetch or read fault, we'll change spte.u=1 and spte.nx=gpte.nx back.
+
+To prevent an spte that was converted into a kernel page with cr0.wp=0
+from being written by the kernel after cr0.wp has changed to 1, we make
+the value of cr0.wp part of the page role.  This means that an spte created
+with one value of cr0.wp cannot be used when cr0.wp has a different value -
+it will simply be missed by the shadow page lookup code.  A similar issue
+exists when an spte created with cr0.wp=0 and cr4.smep=0 is used after
+changing cr4.smep to 1.  To avoid this, the value of !cr0.wp && cr4.smep
+is also made a part of the page role.
+
+Large pages
+===========
+
+The mmu supports all combinations of large and small guest and host pages.
+Supported page sizes include 4k, 2M, 4M, and 1G.  4M pages are treated as
+two separate 2M pages, on both guest and host, since the mmu always uses PAE
+paging.
+
+To instantiate a large spte, four constraints must be satisfied:
+
+- the spte must point to a large host page
+- the guest pte must be a large pte of at least equivalent size (if tdp is
+  enabled, there is no guest pte and this condition is satisfied)
+- if the spte will be writeable, the large page frame may not overlap any
+  write-protected pages
+- the guest page must be wholly contained by a single memory slot
+
+To check the last two conditions, the mmu maintains a ->write_count set of
+arrays for each memory slot and large page size.  Every write protected page
+causes its write_count to be incremented, thus preventing instantiation of
+a large spte.  The frames at the end of an unaligned memory slot have
+artificially inflated ->write_counts so they can never be instantiated.
+
+Zapping all pages (page generation count)
+=========================================
+
+For the large memory guests, walking and zapping all pages is really slow
+(because there are a lot of pages), and also blocks memory accesses of
+all VCPUs because it needs to hold the MMU lock.
+
+To make it be more scalable, kvm maintains a global generation number
+which is stored in kvm->arch.mmu_valid_gen.  Every shadow page stores
+the current global generation-number into sp->mmu_valid_gen when it
+is created.  Pages with a mismatching generation number are "obsolete".
+
+When KVM need zap all shadow pages sptes, it just simply increases the global
+generation-number then reload root shadow pages on all vcpus.  As the VCPUs
+create new shadow page tables, the old pages are not used because of the
+mismatching generation number.
+
+KVM then walks through all pages and zaps obsolete pages.  While the zap
+operation needs to take the MMU lock, the lock can be released periodically
+so that the VCPUs can make progress.
+
+Fast invalidation of MMIO sptes
+===============================
+
+As mentioned in "Reaction to events" above, kvm will cache MMIO
+information in leaf sptes.  When a new memslot is added or an existing
+memslot is changed, this information may become stale and needs to be
+invalidated.  This also needs to hold the MMU lock while walking all
+shadow pages, and is made more scalable with a similar technique.
+
+MMIO sptes have a few spare bits, which are used to store a
+generation number.  The global generation number is stored in
+kvm_memslots(kvm)->generation, and increased whenever guest memory info
+changes.  This generation number is distinct from the one described in
+the previous section.
+
+When KVM finds an MMIO spte, it checks the generation number of the spte.
+If the generation number of the spte does not equal the global generation
+number, it will ignore the cached MMIO information and handle the page
+fault through the slow path.
+
+Since only 19 bits are used to store generation-number on mmio spte, all
+pages are zapped when there is an overflow.
+
+Unfortunately, a single memory access might access kvm_memslots(kvm) multiple
+times, the last one happening when the generation number is retrieved and
+stored into the MMIO spte.  Thus, the MMIO spte might be created based on
+out-of-date information, but with an up-to-date generation number.
+
+To avoid this, the generation number is incremented again after synchronize_srcu
+returns; thus, the low bit of kvm_memslots(kvm)->generation is only 1 during a
+memslot update, while some SRCU readers might be using the old copy.  We do not
+want to use an MMIO sptes created with an odd generation number, and we can do
+this without losing a bit in the MMIO spte.  The low bit of the generation
+is not stored in MMIO spte, and presumed zero when it is extracted out of the
+spte.  If KVM is unlucky and creates an MMIO spte while the low bit is 1,
+the next access to the spte will always be a cache miss.
+
+
+Further reading
+===============
+
+- NPT presentation from KVM Forum 2008
+  http://www.linux-kvm.org/wiki/images/c/c8/KvmForum2008%24kdf2008_21.pdf
+

Documentation/virtual/kvm/msr.txt

@@ -0,0 +1,266 @@
+KVM-specific MSRs.
+Glauber Costa <glommer@redhat.com>, Red Hat Inc, 2010
+=====================================================
+
+KVM makes use of some custom MSRs to service some requests.
+
+Custom MSRs have a range reserved for them, that goes from
+0x4b564d00 to 0x4b564dff. There are MSRs outside this area,
+but they are deprecated and their use is discouraged.
+
+Custom MSR list
+--------
+
+The current supported Custom MSR list is:
+
+MSR_KVM_WALL_CLOCK_NEW:   0x4b564d00
+
+	data: 4-byte alignment physical address of a memory area which must be
+	in guest RAM. This memory is expected to hold a copy of the following
+	structure:
+
+	struct pvclock_wall_clock {
+		u32   version;
+		u32   sec;
+		u32   nsec;
+	} __attribute__((__packed__));
+
+	whose data will be filled in by the hypervisor. The hypervisor is only
+	guaranteed to update this data at the moment of MSR write.
+	Users that want to reliably query this information more than once have
+	to write more than once to this MSR. Fields have the following meanings:
+
+		version: guest has to check version before and after grabbing
+		time information and check that they are both equal and even.
+		An odd version indicates an in-progress update.
+
+		sec: number of seconds for wallclock at time of boot.
+
+		nsec: number of nanoseconds for wallclock at time of boot.
+
+	In order to get the current wallclock time, the system_time from
+	MSR_KVM_SYSTEM_TIME_NEW needs to be added.
+
+	Note that although MSRs are per-CPU entities, the effect of this
+	particular MSR is global.
+
+	Availability of this MSR must be checked via bit 3 in 0x4000001 cpuid
+	leaf prior to usage.
+
+MSR_KVM_SYSTEM_TIME_NEW:  0x4b564d01
+
+	data: 4-byte aligned physical address of a memory area which must be in
+	guest RAM, plus an enable bit in bit 0. This memory is expected to hold
+	a copy of the following structure:
+
+	struct pvclock_vcpu_time_info {
+		u32   version;
+		u32   pad0;
+		u64   tsc_timestamp;
+		u64   system_time;
+		u32   tsc_to_system_mul;
+		s8    tsc_shift;
+		u8    flags;
+		u8    pad[2];
+	} __attribute__((__packed__)); /* 32 bytes */
+
+	whose data will be filled in by the hypervisor periodically. Only one
+	write, or registration, is needed for each VCPU. The interval between
+	updates of this structure is arbitrary and implementation-dependent.
+	The hypervisor may update this structure at any time it sees fit until
+	anything with bit0 == 0 is written to it.
+
+	Fields have the following meanings:
+
+		version: guest has to check version before and after grabbing
+		time information and check that they are both equal and even.
+		An odd version indicates an in-progress update.
+
+		tsc_timestamp: the tsc value at the current VCPU at the time
+		of the update of this structure. Guests can subtract this value
+		from current tsc to derive a notion of elapsed time since the
+		structure update.
+
+		system_time: a host notion of monotonic time, including sleep
+		time at the time this structure was last updated. Unit is
+		nanoseconds.
+
+		tsc_to_system_mul: multiplier to be used when converting
+		tsc-related quantity to nanoseconds
+
+		tsc_shift: shift to be used when converting tsc-related
+		quantity to nanoseconds. This shift will ensure that
+		multiplication with tsc_to_system_mul does not overflow.
+		A positive value denotes a left shift, a negative value
+		a right shift.
+
+		The conversion from tsc to nanoseconds involves an additional
+		right shift by 32 bits. With this information, guests can
+		derive per-CPU time by doing:
+
+			time = (current_tsc - tsc_timestamp)
+			if (tsc_shift >= 0)
+				time <<= tsc_shift;
+			else
+				time >>= -tsc_shift;
+			time = (time * tsc_to_system_mul) >> 32
+			time = time + system_time
+
+		flags: bits in this field indicate extended capabilities
+		coordinated between the guest and the hypervisor. Availability
+		of specific flags has to be checked in 0x40000001 cpuid leaf.
+		Current flags are:
+
+		 flag bit   | cpuid bit    | meaning
+		-------------------------------------------------------------
+			    |	           | time measures taken across
+		     0      |	   24      | multiple cpus are guaranteed to
+			    |		   | be monotonic
+		-------------------------------------------------------------
+			    |		   | guest vcpu has been paused by
+		     1	    |	  N/A	   | the host
+			    |		   | See 4.70 in api.txt
+		-------------------------------------------------------------
+
+	Availability of this MSR must be checked via bit 3 in 0x4000001 cpuid
+	leaf prior to usage.
+
+
+MSR_KVM_WALL_CLOCK:  0x11
+
+	data and functioning: same as MSR_KVM_WALL_CLOCK_NEW. Use that instead.
+
+	This MSR falls outside the reserved KVM range and may be removed in the
+	future. Its usage is deprecated.
+
+	Availability of this MSR must be checked via bit 0 in 0x4000001 cpuid
+	leaf prior to usage.
+
+MSR_KVM_SYSTEM_TIME: 0x12
+
+	data and functioning: same as MSR_KVM_SYSTEM_TIME_NEW. Use that instead.
+
+	This MSR falls outside the reserved KVM range and may be removed in the
+	future. Its usage is deprecated.
+
+	Availability of this MSR must be checked via bit 0 in 0x4000001 cpuid
+	leaf prior to usage.
+
+	The suggested algorithm for detecting kvmclock presence is then:
+
+		if (!kvm_para_available())    /* refer to cpuid.txt */
+			return NON_PRESENT;
+
+		flags = cpuid_eax(0x40000001);
+		if (flags & 3) {
+			msr_kvm_system_time = MSR_KVM_SYSTEM_TIME_NEW;
+			msr_kvm_wall_clock = MSR_KVM_WALL_CLOCK_NEW;
+			return PRESENT;
+		} else if (flags & 0) {
+			msr_kvm_system_time = MSR_KVM_SYSTEM_TIME;
+			msr_kvm_wall_clock = MSR_KVM_WALL_CLOCK;
+			return PRESENT;
+		} else
+			return NON_PRESENT;
+
+MSR_KVM_ASYNC_PF_EN: 0x4b564d02
+	data: Bits 63-6 hold 64-byte aligned physical address of a
+	64 byte memory area which must be in guest RAM and must be
+	zeroed. Bits 5-2 are reserved and should be zero. Bit 0 is 1
+	when asynchronous page faults are enabled on the vcpu 0 when
+	disabled. Bit 2 is 1 if asynchronous page faults can be injected
+	when vcpu is in cpl == 0.
+
+	First 4 byte of 64 byte memory location will be written to by
+	the hypervisor at the time of asynchronous page fault (APF)
+	injection to indicate type of asynchronous page fault. Value
+	of 1 means that the page referred to by the page fault is not
+	present. Value 2 means that the page is now available. Disabling
+	interrupt inhibits APFs. Guest must not enable interrupt
+	before the reason is read, or it may be overwritten by another
+	APF. Since APF uses the same exception vector as regular page
+	fault guest must reset the reason to 0 before it does
+	something that can generate normal page fault.  If during page
+	fault APF reason is 0 it means that this is regular page
+	fault.
+
+	During delivery of type 1 APF cr2 contains a token that will
+	be used to notify a guest when missing page becomes
+	available. When page becomes available type 2 APF is sent with
+	cr2 set to the token associated with the page. There is special
+	kind of token 0xffffffff which tells vcpu that it should wake
+	up all processes waiting for APFs and no individual type 2 APFs
+	will be sent.
+
+	If APF is disabled while there are outstanding APFs, they will
+	not be delivered.
+
+	Currently type 2 APF will be always delivered on the same vcpu as
+	type 1 was, but guest should not rely on that.
+
+MSR_KVM_STEAL_TIME: 0x4b564d03
+
+	data: 64-byte alignment physical address of a memory area which must be
+	in guest RAM, plus an enable bit in bit 0. This memory is expected to
+	hold a copy of the following structure:
+
+	struct kvm_steal_time {
+		__u64 steal;
+		__u32 version;
+		__u32 flags;
+		__u32 pad[12];
+	}
+
+	whose data will be filled in by the hypervisor periodically. Only one
+	write, or registration, is needed for each VCPU. The interval between
+	updates of this structure is arbitrary and implementation-dependent.
+	The hypervisor may update this structure at any time it sees fit until
+	anything with bit0 == 0 is written to it. Guest is required to make sure
+	this structure is initialized to zero.
+
+	Fields have the following meanings:
+
+		version: a sequence counter. In other words, guest has to check
+		this field before and after grabbing time information and make
+		sure they are both equal and even. An odd version indicates an
+		in-progress update.
+
+		flags: At this point, always zero. May be used to indicate
+		changes in this structure in the future.
+
+		steal: the amount of time in which this vCPU did not run, in
+		nanoseconds. Time during which the vcpu is idle, will not be
+		reported as steal time.
+
+MSR_KVM_EOI_EN: 0x4b564d04
+	data: Bit 0 is 1 when PV end of interrupt is enabled on the vcpu; 0
+	when disabled.  Bit 1 is reserved and must be zero.  When PV end of
+	interrupt is enabled (bit 0 set), bits 63-2 hold a 4-byte aligned
+	physical address of a 4 byte memory area which must be in guest RAM and
+	must be zeroed.
+
+	The first, least significant bit of 4 byte memory location will be
+	written to by the hypervisor, typically at the time of interrupt
+	injection.  Value of 1 means that guest can skip writing EOI to the apic
+	(using MSR or MMIO write); instead, it is sufficient to signal
+	EOI by clearing the bit in guest memory - this location will
+	later be polled by the hypervisor.
+	Value of 0 means that the EOI write is required.
+
+	It is always safe for the guest to ignore the optimization and perform
+	the APIC EOI write anyway.
+
+	Hypervisor is guaranteed to only modify this least
+	significant bit while in the current VCPU context, this means that
+	guest does not need to use either lock prefix or memory ordering
+	primitives to synchronise with the hypervisor.
+
+	However, hypervisor can set and clear this memory bit at any time:
+	therefore to make sure hypervisor does not interrupt the
+	guest and clear the least significant bit in the memory area
+	in the window between guest testing it to detect
+	whether it can skip EOI apic write and between guest
+	clearing it to signal EOI to the hypervisor,
+	guest must both read the least significant bit in the memory area and
+	clear it using a single CPU instruction, such as test and clear, or
+	compare and exchange.

Documentation/virtual/kvm/nested-vmx.txt

@@ -0,0 +1,251 @@
+Nested VMX
+==========
+
+Overview
+---------
+
+On Intel processors, KVM uses Intel's VMX (Virtual-Machine eXtensions)
+to easily and efficiently run guest operating systems. Normally, these guests
+*cannot* themselves be hypervisors running their own guests, because in VMX,
+guests cannot use VMX instructions.
+
+The "Nested VMX" feature adds this missing capability - of running guest
+hypervisors (which use VMX) with their own nested guests. It does so by
+allowing a guest to use VMX instructions, and correctly and efficiently
+emulating them using the single level of VMX available in the hardware.
+
+We describe in much greater detail the theory behind the nested VMX feature,
+its implementation and its performance characteristics, in the OSDI 2010 paper
+"The Turtles Project: Design and Implementation of Nested Virtualization",
+available at:
+
+	http://www.usenix.org/events/osdi10/tech/full_papers/Ben-Yehuda.pdf
+
+
+Terminology
+-----------
+
+Single-level virtualization has two levels - the host (KVM) and the guests.
+In nested virtualization, we have three levels: The host (KVM), which we call
+L0, the guest hypervisor, which we call L1, and its nested guest, which we
+call L2.
+
+
+Known limitations
+-----------------
+
+The current code supports running Linux guests under KVM guests.
+Only 64-bit guest hypervisors are supported.
+
+Additional patches for running Windows under guest KVM, and Linux under
+guest VMware server, and support for nested EPT, are currently running in
+the lab, and will be sent as follow-on patchsets.
+
+
+Running nested VMX
+------------------
+
+The nested VMX feature is disabled by default. It can be enabled by giving
+the "nested=1" option to the kvm-intel module.
+
+No modifications are required to user space (qemu). However, qemu's default
+emulated CPU type (qemu64) does not list the "VMX" CPU feature, so it must be
+explicitly enabled, by giving qemu one of the following options:
+
+     -cpu host              (emulated CPU has all features of the real CPU)
+
+     -cpu qemu64,+vmx       (add just the vmx feature to a named CPU type)
+
+
+ABIs
+----
+
+Nested VMX aims to present a standard and (eventually) fully-functional VMX
+implementation for the a guest hypervisor to use. As such, the official
+specification of the ABI that it provides is Intel's VMX specification,
+namely volume 3B of their "Intel 64 and IA-32 Architectures Software
+Developer's Manual". Not all of VMX's features are currently fully supported,
+but the goal is to eventually support them all, starting with the VMX features
+which are used in practice by popular hypervisors (KVM and others).
+
+As a VMX implementation, nested VMX presents a VMCS structure to L1.
+As mandated by the spec, other than the two fields revision_id and abort,
+this structure is *opaque* to its user, who is not supposed to know or care
+about its internal structure. Rather, the structure is accessed through the
+VMREAD and VMWRITE instructions.
+Still, for debugging purposes, KVM developers might be interested to know the
+internals of this structure; This is struct vmcs12 from arch/x86/kvm/vmx.c.
+
+The name "vmcs12" refers to the VMCS that L1 builds for L2. In the code we
+also have "vmcs01", the VMCS that L0 built for L1, and "vmcs02" is the VMCS
+which L0 builds to actually run L2 - how this is done is explained in the
+aforementioned paper.
+
+For convenience, we repeat the content of struct vmcs12 here. If the internals
+of this structure changes, this can break live migration across KVM versions.
+VMCS12_REVISION (from vmx.c) should be changed if struct vmcs12 or its inner
+struct shadow_vmcs is ever changed.
+
+	typedef u64 natural_width;
+	struct __packed vmcs12 {
+		/* According to the Intel spec, a VMCS region must start with
+		 * these two user-visible fields */
+		u32 revision_id;
+		u32 abort;
+
+		u32 launch_state; /* set to 0 by VMCLEAR, to 1 by VMLAUNCH */
+		u32 padding[7]; /* room for future expansion */
+
+		u64 io_bitmap_a;
+		u64 io_bitmap_b;
+		u64 msr_bitmap;
+		u64 vm_exit_msr_store_addr;
+		u64 vm_exit_msr_load_addr;
+		u64 vm_entry_msr_load_addr;
+		u64 tsc_offset;
+		u64 virtual_apic_page_addr;
+		u64 apic_access_addr;
+		u64 ept_pointer;
+		u64 guest_physical_address;
+		u64 vmcs_link_pointer;
+		u64 guest_ia32_debugctl;
+		u64 guest_ia32_pat;
+		u64 guest_ia32_efer;
+		u64 guest_pdptr0;
+		u64 guest_pdptr1;
+		u64 guest_pdptr2;
+		u64 guest_pdptr3;
+		u64 host_ia32_pat;
+		u64 host_ia32_efer;
+		u64 padding64[8]; /* room for future expansion */
+		natural_width cr0_guest_host_mask;
+		natural_width cr4_guest_host_mask;
+		natural_width cr0_read_shadow;
+		natural_width cr4_read_shadow;
+		natural_width cr3_target_value0;
+		natural_width cr3_target_value1;
+		natural_width cr3_target_value2;
+		natural_width cr3_target_value3;
+		natural_width exit_qualification;
+		natural_width guest_linear_address;
+		natural_width guest_cr0;
+		natural_width guest_cr3;
+		natural_width guest_cr4;
+		natural_width guest_es_base;
+		natural_width guest_cs_base;
+		natural_width guest_ss_base;
+		natural_width guest_ds_base;
+		natural_width guest_fs_base;
+		natural_width guest_gs_base;
+		natural_width guest_ldtr_base;
+		natural_width guest_tr_base;
+		natural_width guest_gdtr_base;
+		natural_width guest_idtr_base;
+		natural_width guest_dr7;
+		natural_width guest_rsp;
+		natural_width guest_rip;
+		natural_width guest_rflags;
+		natural_width guest_pending_dbg_exceptions;
+		natural_width guest_sysenter_esp;
+		natural_width guest_sysenter_eip;
+		natural_width host_cr0;
+		natural_width host_cr3;
+		natural_width host_cr4;
+		natural_width host_fs_base;
+		natural_width host_gs_base;
+		natural_width host_tr_base;
+		natural_width host_gdtr_base;
+		natural_width host_idtr_base;
+		natural_width host_ia32_sysenter_esp;
+		natural_width host_ia32_sysenter_eip;
+		natural_width host_rsp;
+		natural_width host_rip;
+		natural_width paddingl[8]; /* room for future expansion */
+		u32 pin_based_vm_exec_control;
+		u32 cpu_based_vm_exec_control;
+		u32 exception_bitmap;
+		u32 page_fault_error_code_mask;
+		u32 page_fault_error_code_match;
+		u32 cr3_target_count;
+		u32 vm_exit_controls;
+		u32 vm_exit_msr_store_count;
+		u32 vm_exit_msr_load_count;
+		u32 vm_entry_controls;
+		u32 vm_entry_msr_load_count;
+		u32 vm_entry_intr_info_field;
+		u32 vm_entry_exception_error_code;
+		u32 vm_entry_instruction_len;
+		u32 tpr_threshold;
+		u32 secondary_vm_exec_control;
+		u32 vm_instruction_error;
+		u32 vm_exit_reason;
+		u32 vm_exit_intr_info;
+		u32 vm_exit_intr_error_code;
+		u32 idt_vectoring_info_field;
+		u32 idt_vectoring_error_code;
+		u32 vm_exit_instruction_len;
+		u32 vmx_instruction_info;
+		u32 guest_es_limit;
+		u32 guest_cs_limit;
+		u32 guest_ss_limit;
+		u32 guest_ds_limit;
+		u32 guest_fs_limit;
+		u32 guest_gs_limit;
+		u32 guest_ldtr_limit;
+		u32 guest_tr_limit;
+		u32 guest_gdtr_limit;
+		u32 guest_idtr_limit;
+		u32 guest_es_ar_bytes;
+		u32 guest_cs_ar_bytes;
+		u32 guest_ss_ar_bytes;
+		u32 guest_ds_ar_bytes;
+		u32 guest_fs_ar_bytes;
+		u32 guest_gs_ar_bytes;
+		u32 guest_ldtr_ar_bytes;
+		u32 guest_tr_ar_bytes;
+		u32 guest_interruptibility_info;
+		u32 guest_activity_state;
+		u32 guest_sysenter_cs;
+		u32 host_ia32_sysenter_cs;
+		u32 padding32[8]; /* room for future expansion */
+		u16 virtual_processor_id;
+		u16 guest_es_selector;
+		u16 guest_cs_selector;
+		u16 guest_ss_selector;
+		u16 guest_ds_selector;
+		u16 guest_fs_selector;
+		u16 guest_gs_selector;
+		u16 guest_ldtr_selector;
+		u16 guest_tr_selector;
+		u16 host_es_selector;
+		u16 host_cs_selector;
+		u16 host_ss_selector;
+		u16 host_ds_selector;
+		u16 host_fs_selector;
+		u16 host_gs_selector;
+		u16 host_tr_selector;
+	};
+
+
+Authors
+-------
+
+These patches were written by:
+     Abel Gordon, abelg <at> il.ibm.com
+     Nadav Har'El, nyh <at> il.ibm.com
+     Orit Wasserman, oritw <at> il.ibm.com
+     Ben-Ami Yassor, benami <at> il.ibm.com
+     Muli Ben-Yehuda, muli <at> il.ibm.com
+
+With contributions by:
+     Anthony Liguori, aliguori <at> us.ibm.com
+     Mike Day, mdday <at> us.ibm.com
+     Michael Factor, factor <at> il.ibm.com
+     Zvi Dubitzky, dubi <at> il.ibm.com
+
+And valuable reviews by:
+     Avi Kivity, avi <at> redhat.com
+     Gleb Natapov, gleb <at> redhat.com
+     Marcelo Tosatti, mtosatti <at> redhat.com
+     Kevin Tian, kevin.tian <at> intel.com
+     and others.

Documentation/virtual/kvm/ppc-pv.txt

@@ -0,0 +1,212 @@
+The PPC KVM paravirtual interface
+=================================
+
+The basic execution principle by which KVM on PowerPC works is to run all kernel
+space code in PR=1 which is user space. This way we trap all privileged
+instructions and can emulate them accordingly.
+
+Unfortunately that is also the downfall. There are quite some privileged
+instructions that needlessly return us to the hypervisor even though they
+could be handled differently.
+
+This is what the PPC PV interface helps with. It takes privileged instructions
+and transforms them into unprivileged ones with some help from the hypervisor.
+This cuts down virtualization costs by about 50% on some of my benchmarks.
+
+The code for that interface can be found in arch/powerpc/kernel/kvm*
+
+Querying for existence
+======================
+
+To find out if we're running on KVM or not, we leverage the device tree. When
+Linux is running on KVM, a node /hypervisor exists. That node contains a
+compatible property with the value "linux,kvm".
+
+Once you determined you're running under a PV capable KVM, you can now use
+hypercalls as described below.
+
+KVM hypercalls
+==============
+
+Inside the device tree's /hypervisor node there's a property called
+'hypercall-instructions'. This property contains at most 4 opcodes that make
+up the hypercall. To call a hypercall, just call these instructions.
+
+The parameters are as follows:
+
+	Register	IN			OUT
+
+	r0		-			volatile
+	r3		1st parameter		Return code
+	r4		2nd parameter		1st output value
+	r5		3rd parameter		2nd output value
+	r6		4th parameter		3rd output value
+	r7		5th parameter		4th output value
+	r8		6th parameter		5th output value
+	r9		7th parameter		6th output value
+	r10		8th parameter		7th output value
+	r11		hypercall number	8th output value
+	r12		-			volatile
+
+Hypercall definitions are shared in generic code, so the same hypercall numbers
+apply for x86 and powerpc alike with the exception that each KVM hypercall
+also needs to be ORed with the KVM vendor code which is (42 << 16).
+
+Return codes can be as follows:
+
+	Code		Meaning
+
+	0		Success
+	12		Hypercall not implemented
+	<0		Error
+
+The magic page
+==============
+
+To enable communication between the hypervisor and guest there is a new shared
+page that contains parts of supervisor visible register state. The guest can
+map this shared page using the KVM hypercall KVM_HC_PPC_MAP_MAGIC_PAGE.
+
+With this hypercall issued the guest always gets the magic page mapped at the
+desired location. The first parameter indicates the effective address when the
+MMU is enabled. The second parameter indicates the address in real mode, if
+applicable to the target. For now, we always map the page to -4096. This way we
+can access it using absolute load and store functions. The following
+instruction reads the first field of the magic page:
+
+	ld	rX, -4096(0)
+
+The interface is designed to be extensible should there be need later to add
+additional registers to the magic page. If you add fields to the magic page,
+also define a new hypercall feature to indicate that the host can give you more
+registers. Only if the host supports the additional features, make use of them.
+
+The magic page layout is described by struct kvm_vcpu_arch_shared
+in arch/powerpc/include/asm/kvm_para.h.
+
+Magic page features
+===================
+
+When mapping the magic page using the KVM hypercall KVM_HC_PPC_MAP_MAGIC_PAGE,
+a second return value is passed to the guest. This second return value contains
+a bitmap of available features inside the magic page.
+
+The following enhancements to the magic page are currently available:
+
+  KVM_MAGIC_FEAT_SR		Maps SR registers r/w in the magic page
+  KVM_MAGIC_FEAT_MAS0_TO_SPRG7	Maps MASn, ESR, PIR and high SPRGs
+
+For enhanced features in the magic page, please check for the existence of the
+feature before using them!
+
+Magic page flags
+================
+
+In addition to features that indicate whether a host is capable of a particular
+feature we also have a channel for a guest to tell the guest whether it's capable
+of something. This is what we call "flags".
+
+Flags are passed to the host in the low 12 bits of the Effective Address.
+
+The following flags are currently available for a guest to expose:
+
+  MAGIC_PAGE_FLAG_NOT_MAPPED_NX Guest handles NX bits correclty wrt magic page
+
+MSR bits
+========
+
+The MSR contains bits that require hypervisor intervention and bits that do
+not require direct hypervisor intervention because they only get interpreted
+when entering the guest or don't have any impact on the hypervisor's behavior.
+
+The following bits are safe to be set inside the guest:
+
+  MSR_EE
+  MSR_RI
+
+If any other bit changes in the MSR, please still use mtmsr(d).
+
+Patched instructions
+====================
+
+The "ld" and "std" instructions are transformed to "lwz" and "stw" instructions
+respectively on 32 bit systems with an added offset of 4 to accommodate for big
+endianness.
+
+The following is a list of mapping the Linux kernel performs when running as
+guest. Implementing any of those mappings is optional, as the instruction traps
+also act on the shared page. So calling privileged instructions still works as
+before.
+
+From			To
+====			==
+
+mfmsr	rX		ld	rX, magic_page->msr
+mfsprg	rX, 0		ld	rX, magic_page->sprg0
+mfsprg	rX, 1		ld	rX, magic_page->sprg1
+mfsprg	rX, 2		ld	rX, magic_page->sprg2
+mfsprg	rX, 3		ld	rX, magic_page->sprg3
+mfsrr0	rX		ld	rX, magic_page->srr0
+mfsrr1	rX		ld	rX, magic_page->srr1
+mfdar	rX		ld	rX, magic_page->dar
+mfdsisr	rX		lwz	rX, magic_page->dsisr
+
+mtmsr	rX		std	rX, magic_page->msr
+mtsprg	0, rX		std	rX, magic_page->sprg0
+mtsprg	1, rX		std	rX, magic_page->sprg1
+mtsprg	2, rX		std	rX, magic_page->sprg2
+mtsprg	3, rX		std	rX, magic_page->sprg3
+mtsrr0	rX		std	rX, magic_page->srr0
+mtsrr1	rX		std	rX, magic_page->srr1
+mtdar	rX		std	rX, magic_page->dar
+mtdsisr	rX		stw	rX, magic_page->dsisr
+
+tlbsync			nop
+
+mtmsrd	rX, 0		b	<special mtmsr section>
+mtmsr	rX		b	<special mtmsr section>
+
+mtmsrd	rX, 1		b	<special mtmsrd section>
+
+[Book3S only]
+mtsrin	rX, rY		b	<special mtsrin section>
+
+[BookE only]
+wrteei	[0|1]		b	<special wrteei section>
+
+
+Some instructions require more logic to determine what's going on than a load
+or store instruction can deliver. To enable patching of those, we keep some
+RAM around where we can live translate instructions to. What happens is the
+following:
+
+	1) copy emulation code to memory
+	2) patch that code to fit the emulated instruction
+	3) patch that code to return to the original pc + 4
+	4) patch the original instruction to branch to the new code
+
+That way we can inject an arbitrary amount of code as replacement for a single
+instruction. This allows us to check for pending interrupts when setting EE=1
+for example.
+
+Hypercall ABIs in KVM on PowerPC
+=================================
+1) KVM hypercalls (ePAPR)
+
+These are ePAPR compliant hypercall implementation (mentioned above). Even
+generic hypercalls are implemented here, like the ePAPR idle hcall. These are
+available on all targets.
+
+2) PAPR hypercalls
+
+PAPR hypercalls are needed to run server PowerPC PAPR guests (-M pseries in QEMU).
+These are the same hypercalls that pHyp, the POWER hypervisor implements. Some of
+them are handled in the kernel, some are handled in user space. This is only
+available on book3s_64.
+
+3) OSI hypercalls
+
+Mac-on-Linux is another user of KVM on PowerPC, which has its own hypercall (long
+before KVM). This is supported to maintain compatibility. All these hypercalls get
+forwarded to user space. This is only useful on book3s_32, but can be used with
+book3s_64 as well.

Documentation/virtual/kvm/review-checklist.txt

@@ -0,0 +1,38 @@
+Review checklist for kvm patches
+================================
+
+1.  The patch must follow Documentation/CodingStyle and
+    Documentation/SubmittingPatches.
+
+2.  Patches should be against kvm.git master branch.
+
+3.  If the patch introduces or modifies a new userspace API:
+    - the API must be documented in Documentation/virtual/kvm/api.txt
+    - the API must be discoverable using KVM_CHECK_EXTENSION
+
+4.  New state must include support for save/restore.
+
+5.  New features must default to off (userspace should explicitly request them).
+    Performance improvements can and should default to on.
+
+6.  New cpu features should be exposed via KVM_GET_SUPPORTED_CPUID2
+
+7.  Emulator changes should be accompanied by unit tests for qemu-kvm.git
+    kvm/test directory.
+
+8.  Changes should be vendor neutral when possible.  Changes to common code
+    are better than duplicating changes to vendor code.
+
+9.  Similarly, prefer changes to arch independent code than to arch dependent
+    code.
+
+10. User/kernel interfaces and guest/host interfaces must be 64-bit clean
+    (all variables and sizes naturally aligned on 64-bit; use specific types
+    only - u64 rather than ulong).
+
+11. New guest visible features must either be documented in a hardware manual
+    or be accompanied by documentation.
+
+12. Features must be robust against reset and kexec - for example, shared
+    host/guest memory must be unshared to prevent the host from writing to
+    guest memory that the guest has not reserved for this purpose.

Documentation/virtual/kvm/s390-diag.txt

@@ -0,0 +1,82 @@
+The s390 DIAGNOSE call on KVM
+=============================
+
+KVM on s390 supports the DIAGNOSE call for making hypercalls, both for
+native hypercalls and for selected hypercalls found on other s390
+hypervisors.
+
+Note that bits are numbered as by the usual s390 convention (most significant
+bit on the left).
+
+
+General remarks
+---------------
+
+DIAGNOSE calls by the guest cause a mandatory intercept. This implies
+all supported DIAGNOSE calls need to be handled by either KVM or its
+userspace.
+
+All DIAGNOSE calls supported by KVM use the RS-a format:
+
+--------------------------------------
+|  '83'  | R1 | R3 | B2 |     D2     |
+--------------------------------------
+0        8    12   16   20           31
+
+The second-operand address (obtained by the base/displacement calculation)
+is not used to address data. Instead, bits 48-63 of this address specify
+the function code, and bits 0-47 are ignored.
+
+The supported DIAGNOSE function codes vary by the userspace used. For
+DIAGNOSE function codes not specific to KVM, please refer to the
+documentation for the s390 hypervisors defining them.
+
+
+DIAGNOSE function code 'X'500' - KVM virtio functions
+-----------------------------------------------------
+
+If the function code specifies 0x500, various virtio-related functions
+are performed.
+
+General register 1 contains the virtio subfunction code. Supported
+virtio subfunctions depend on KVM's userspace. Generally, userspace
+provides either s390-virtio (subcodes 0-2) or virtio-ccw (subcode 3).
+
+Upon completion of the DIAGNOSE instruction, general register 2 contains
+the function's return code, which is either a return code or a subcode
+specific value.
+
+Subcode 0 - s390-virtio notification and early console printk
+    Handled by userspace.
+
+Subcode 1 - s390-virtio reset
+    Handled by userspace.
+
+Subcode 2 - s390-virtio set status
+    Handled by userspace.
+
+Subcode 3 - virtio-ccw notification
+    Handled by either userspace or KVM (ioeventfd case).
+
+    General register 2 contains a subchannel-identification word denoting
+    the subchannel of the virtio-ccw proxy device to be notified.
+
+    General register 3 contains the number of the virtqueue to be notified.
+
+    General register 4 contains a 64bit identifier for KVM usage (the
+    kvm_io_bus cookie). If general register 4 does not contain a valid
+    identifier, it is ignored.
+
+    After completion of the DIAGNOSE call, general register 2 may contain
+    a 64bit identifier (in the kvm_io_bus cookie case).
+
+    See also the virtio standard for a discussion of this hypercall.
+
+
+DIAGNOSE function code 'X'501 - KVM breakpoint
+----------------------------------------------
+
+If the function code specifies 0x501, breakpoint functions may be performed.
+This function code is handled by userspace.
+
+This diagnose function code has no subfunctions and uses no parameters.

Documentation/virtual/kvm/timekeeping.txt

@@ -0,0 +1,612 @@
+
+	Timekeeping Virtualization for X86-Based Architectures
+
+	Zachary Amsden <zamsden@redhat.com>
+	Copyright (c) 2010, Red Hat.  All rights reserved.
+
+1) Overview
+2) Timing Devices
+3) TSC Hardware
+4) Virtualization Problems
+
+=========================================================================
+
+1) Overview
+
+One of the most complicated parts of the X86 platform, and specifically,
+the virtualization of this platform is the plethora of timing devices available
+and the complexity of emulating those devices.  In addition, virtualization of
+time introduces a new set of challenges because it introduces a multiplexed
+division of time beyond the control of the guest CPU.
+
+First, we will describe the various timekeeping hardware available, then
+present some of the problems which arise and solutions available, giving
+specific recommendations for certain classes of KVM guests.
+
+The purpose of this document is to collect data and information relevant to
+timekeeping which may be difficult to find elsewhere, specifically,
+information relevant to KVM and hardware-based virtualization.
+
+=========================================================================
+
+2) Timing Devices
+
+First we discuss the basic hardware devices available.  TSC and the related
+KVM clock are special enough to warrant a full exposition and are described in
+the following section.
+
+2.1) i8254 - PIT
+
+One of the first timer devices available is the programmable interrupt timer,
+or PIT.  The PIT has a fixed frequency 1.193182 MHz base clock and three
+channels which can be programmed to deliver periodic or one-shot interrupts.
+These three channels can be configured in different modes and have individual
+counters.  Channel 1 and 2 were not available for general use in the original
+IBM PC, and historically were connected to control RAM refresh and the PC
+speaker.  Now the PIT is typically integrated as part of an emulated chipset
+and a separate physical PIT is not used.
+
+The PIT uses I/O ports 0x40 - 0x43.  Access to the 16-bit counters is done
+using single or multiple byte access to the I/O ports.  There are 6 modes
+available, but not all modes are available to all timers, as only timer 2
+has a connected gate input, required for modes 1 and 5.  The gate line is
+controlled by port 61h, bit 0, as illustrated in the following diagram.
+
+ --------------             ----------------
+|              |           |                |
+|  1.1932 MHz  |---------->| CLOCK      OUT | ---------> IRQ 0
+|    Clock     |   |       |                |
+ --------------    |    +->| GATE  TIMER 0  |
+                   |        ----------------
+                   |
+                   |        ----------------
+                   |       |                |
+                   |------>| CLOCK      OUT | ---------> 66.3 KHZ DRAM
+                   |       |                |            (aka /dev/null)
+                   |    +->| GATE  TIMER 1  |
+                   |        ----------------
+                   |
+                   |        ----------------
+                   |       |                |
+                   |------>| CLOCK      OUT | ---------> Port 61h, bit 5
+                           |                |      |
+Port 61h, bit 0 ---------->| GATE  TIMER 2  |       \_.----   ____
+                            ----------------         _|    )--|LPF|---Speaker
+                                                    / *----   \___/
+Port 61h, bit 1 -----------------------------------/
+
+The timer modes are now described.
+
+Mode 0: Single Timeout.   This is a one-shot software timeout that counts down
+ when the gate is high (always true for timers 0 and 1).  When the count
+ reaches zero, the output goes high.
+
+Mode 1: Triggered One-shot.  The output is initially set high.  When the gate
+ line is set high, a countdown is initiated (which does not stop if the gate is
+ lowered), during which the output is set low.  When the count reaches zero,
+ the output goes high.
+
+Mode 2: Rate Generator.  The output is initially set high.  When the countdown
+ reaches 1, the output goes low for one count and then returns high.  The value
+ is reloaded and the countdown automatically resumes.  If the gate line goes
+ low, the count is halted.  If the output is low when the gate is lowered, the
+ output automatically goes high (this only affects timer 2).
+
+Mode 3: Square Wave.   This generates a high / low square wave.  The count
+ determines the length of the pulse, which alternates between high and low
+ when zero is reached.  The count only proceeds when gate is high and is
+ automatically reloaded on reaching zero.  The count is decremented twice at
+ each clock to generate a full high / low cycle at the full periodic rate.
+ If the count is even, the clock remains high for N/2 counts and low for N/2
+ counts; if the clock is odd, the clock is high for (N+1)/2 counts and low
+ for (N-1)/2 counts.  Only even values are latched by the counter, so odd
+ values are not observed when reading.  This is the intended mode for timer 2,
+ which generates sine-like tones by low-pass filtering the square wave output.
+
+Mode 4: Software Strobe.  After programming this mode and loading the counter,
+ the output remains high until the counter reaches zero.  Then the output
+ goes low for 1 clock cycle and returns high.  The counter is not reloaded.
+ Counting only occurs when gate is high.
+
+Mode 5: Hardware Strobe.  After programming and loading the counter, the
+ output remains high.  When the gate is raised, a countdown is initiated
+ (which does not stop if the gate is lowered).  When the counter reaches zero,
+ the output goes low for 1 clock cycle and then returns high.  The counter is
+ not reloaded.
+
+In addition to normal binary counting, the PIT supports BCD counting.  The
+command port, 0x43 is used to set the counter and mode for each of the three
+timers.
+
+PIT commands, issued to port 0x43, using the following bit encoding:
+
+Bit 7-4: Command (See table below)
+Bit 3-1: Mode (000 = Mode 0, 101 = Mode 5, 11X = undefined)
+Bit 0  : Binary (0) / BCD (1)
+
+Command table:
+
+0000 - Latch Timer 0 count for port 0x40
+	sample and hold the count to be read in port 0x40;
+	additional commands ignored until counter is read;
+	mode bits ignored.
+
+0001 - Set Timer 0 LSB mode for port 0x40
+	set timer to read LSB only and force MSB to zero;
+	mode bits set timer mode
+
+0010 - Set Timer 0 MSB mode for port 0x40
+	set timer to read MSB only and force LSB to zero;
+	mode bits set timer mode
+
+0011 - Set Timer 0 16-bit mode for port 0x40
+	set timer to read / write LSB first, then MSB;
+	mode bits set timer mode
+
+0100 - Latch Timer 1 count for port 0x41 - as described above
+0101 - Set Timer 1 LSB mode for port 0x41 - as described above
+0110 - Set Timer 1 MSB mode for port 0x41 - as described above
+0111 - Set Timer 1 16-bit mode for port 0x41 - as described above
+
+1000 - Latch Timer 2 count for port 0x42 - as described above
+1001 - Set Timer 2 LSB mode for port 0x42 - as described above
+1010 - Set Timer 2 MSB mode for port 0x42 - as described above
+1011 - Set Timer 2 16-bit mode for port 0x42 as described above
+
+1101 - General counter latch
+	Latch combination of counters into corresponding ports
+	Bit 3 = Counter 2
+	Bit 2 = Counter 1
+	Bit 1 = Counter 0
+	Bit 0 = Unused
+
+1110 - Latch timer status
+	Latch combination of counter mode into corresponding ports
+	Bit 3 = Counter 2
+	Bit 2 = Counter 1
+	Bit 1 = Counter 0
+
+	The output of ports 0x40-0x42 following this command will be:
+
+	Bit 7 = Output pin
+	Bit 6 = Count loaded (0 if timer has expired)
+	Bit 5-4 = Read / Write mode
+	    01 = MSB only
+	    10 = LSB only
+	    11 = LSB / MSB (16-bit)
+	Bit 3-1 = Mode
+	Bit 0 = Binary (0) / BCD mode (1)
+
+2.2) RTC
+
+The second device which was available in the original PC was the MC146818 real
+time clock.  The original device is now obsolete, and usually emulated by the
+system chipset, sometimes by an HPET and some frankenstein IRQ routing.
+
+The RTC is accessed through CMOS variables, which uses an index register to
+control which bytes are read.  Since there is only one index register, read
+of the CMOS and read of the RTC require lock protection (in addition, it is
+dangerous to allow userspace utilities such as hwclock to have direct RTC
+access, as they could corrupt kernel reads and writes of CMOS memory).
+
+The RTC generates an interrupt which is usually routed to IRQ 8.  The interrupt
+can function as a periodic timer, an additional once a day alarm, and can issue
+interrupts after an update of the CMOS registers by the MC146818 is complete.
+The type of interrupt is signalled in the RTC status registers.
+
+The RTC will update the current time fields by battery power even while the
+system is off.  The current time fields should not be read while an update is
+in progress, as indicated in the status register.
+
+The clock uses a 32.768kHz crystal, so bits 6-4 of register A should be
+programmed to a 32kHz divider if the RTC is to count seconds.
+
+This is the RAM map originally used for the RTC/CMOS:
+
+Location    Size    Description
+------------------------------------------
+00h         byte    Current second (BCD)
+01h         byte    Seconds alarm (BCD)
+02h         byte    Current minute (BCD)
+03h         byte    Minutes alarm (BCD)
+04h         byte    Current hour (BCD)
+05h         byte    Hours alarm (BCD)
+06h         byte    Current day of week (BCD)
+07h         byte    Current day of month (BCD)
+08h         byte    Current month (BCD)
+09h         byte    Current year (BCD)
+0Ah         byte    Register A
+                       bit 7   = Update in progress
+                       bit 6-4 = Divider for clock
+                                  000 = 4.194 MHz
+                                  001 = 1.049 MHz
+                                  010 = 32 kHz
+                                  10X = test modes
+                                  110 = reset / disable
+                                  111 = reset / disable
+                       bit 3-0 = Rate selection for periodic interrupt
+                                  000 = periodic timer disabled
+                                  001 = 3.90625 uS
+                                  010 = 7.8125 uS
+                                  011 = .122070 mS
+                                  100 = .244141 mS
+                                     ...
+                                 1101 = 125 mS
+                                 1110 = 250 mS
+                                 1111 = 500 mS
+0Bh         byte    Register B
+                       bit 7   = Run (0) / Halt (1)
+                       bit 6   = Periodic interrupt enable
+                       bit 5   = Alarm interrupt enable
+                       bit 4   = Update-ended interrupt enable
+                       bit 3   = Square wave interrupt enable
+                       bit 2   = BCD calendar (0) / Binary (1)
+                       bit 1   = 12-hour mode (0) / 24-hour mode (1)
+                       bit 0   = 0 (DST off) / 1 (DST enabled)
+OCh         byte    Register C (read only)
+                       bit 7   = interrupt request flag (IRQF)
+                       bit 6   = periodic interrupt flag (PF)
+                       bit 5   = alarm interrupt flag (AF)
+                       bit 4   = update interrupt flag (UF)
+                       bit 3-0 = reserved
+ODh         byte    Register D (read only)
+                       bit 7   = RTC has power
+                       bit 6-0 = reserved
+32h         byte    Current century BCD (*)
+  (*) location vendor specific and now determined from ACPI global tables
+
+2.3) APIC
+
+On Pentium and later processors, an on-board timer is available to each CPU
+as part of the Advanced Programmable Interrupt Controller.  The APIC is
+accessed through memory-mapped registers and provides interrupt service to each
+CPU, used for IPIs and local timer interrupts.
+
+Although in theory the APIC is a safe and stable source for local interrupts,
+in practice, many bugs and glitches have occurred due to the special nature of
+the APIC CPU-local memory-mapped hardware.  Beware that CPU errata may affect
+the use of the APIC and that workarounds may be required.  In addition, some of
+these workarounds pose unique constraints for virtualization - requiring either
+extra overhead incurred from extra reads of memory-mapped I/O or additional
+functionality that may be more computationally expensive to implement.
+
+Since the APIC is documented quite well in the Intel and AMD manuals, we will
+avoid repetition of the detail here.  It should be pointed out that the APIC
+timer is programmed through the LVT (local vector timer) register, is capable
+of one-shot or periodic operation, and is based on the bus clock divided down
+by the programmable divider register.
+
+2.4) HPET
+
+HPET is quite complex, and was originally intended to replace the PIT / RTC
+support of the X86 PC.  It remains to be seen whether that will be the case, as
+the de facto standard of PC hardware is to emulate these older devices.  Some
+systems designated as legacy free may support only the HPET as a hardware timer
+device.
+
+The HPET spec is rather loose and vague, requiring at least 3 hardware timers,
+but allowing implementation freedom to support many more.  It also imposes no
+fixed rate on the timer frequency, but does impose some extremal values on
+frequency, error and slew.
+
+In general, the HPET is recommended as a high precision (compared to PIT /RTC)
+time source which is independent of local variation (as there is only one HPET
+in any given system).  The HPET is also memory-mapped, and its presence is
+indicated through ACPI tables by the BIOS.
+
+Detailed specification of the HPET is beyond the current scope of this
+document, as it is also very well documented elsewhere.
+
+2.5) Offboard Timers
+
+Several cards, both proprietary (watchdog boards) and commonplace (e1000) have
+timing chips built into the cards which may have registers which are accessible
+to kernel or user drivers.  To the author's knowledge, using these to generate
+a clocksource for a Linux or other kernel has not yet been attempted and is in
+general frowned upon as not playing by the agreed rules of the game.  Such a
+timer device would require additional support to be virtualized properly and is
+not considered important at this time as no known operating system does this.
+
+=========================================================================
+
+3) TSC Hardware
+
+The TSC or time stamp counter is relatively simple in theory; it counts
+instruction cycles issued by the processor, which can be used as a measure of
+time.  In practice, due to a number of problems, it is the most complicated
+timekeeping device to use.
+
+The TSC is represented internally as a 64-bit MSR which can be read with the
+RDMSR, RDTSC, or RDTSCP (when available) instructions.  In the past, hardware
+limitations made it possible to write the TSC, but generally on old hardware it
+was only possible to write the low 32-bits of the 64-bit counter, and the upper
+32-bits of the counter were cleared.  Now, however, on Intel processors family
+0Fh, for models 3, 4 and 6, and family 06h, models e and f, this restriction
+has been lifted and all 64-bits are writable.  On AMD systems, the ability to
+write the TSC MSR is not an architectural guarantee.
+
+The TSC is accessible from CPL-0 and conditionally, for CPL > 0 software by
+means of the CR4.TSD bit, which when enabled, disables CPL > 0 TSC access.
+
+Some vendors have implemented an additional instruction, RDTSCP, which returns
+atomically not just the TSC, but an indicator which corresponds to the
+processor number.  This can be used to index into an array of TSC variables to
+determine offset information in SMP systems where TSCs are not synchronized.
+The presence of this instruction must be determined by consulting CPUID feature
+bits.
+
+Both VMX and SVM provide extension fields in the virtualization hardware which
+allows the guest visible TSC to be offset by a constant.  Newer implementations
+promise to allow the TSC to additionally be scaled, but this hardware is not
+yet widely available.
+
+3.1) TSC synchronization
+
+The TSC is a CPU-local clock in most implementations.  This means, on SMP
+platforms, the TSCs of different CPUs may start at different times depending
+on when the CPUs are powered on.  Generally, CPUs on the same die will share
+the same clock, however, this is not always the case.
+
+The BIOS may attempt to resynchronize the TSCs during the poweron process and
+the operating system or other system software may attempt to do this as well.
+Several hardware limitations make the problem worse - if it is not possible to
+write the full 64-bits of the TSC, it may be impossible to match the TSC in
+newly arriving CPUs to that of the rest of the system, resulting in
+unsynchronized TSCs.  This may be done by BIOS or system software, but in
+practice, getting a perfectly synchronized TSC will not be possible unless all
+values are read from the same clock, which generally only is possible on single
+socket systems or those with special hardware support.
+
+3.2) TSC and CPU hotplug
+
+As touched on already, CPUs which arrive later than the boot time of the system
+may not have a TSC value that is synchronized with the rest of the system.
+Either system software, BIOS, or SMM code may actually try to establish the TSC
+to a value matching the rest of the system, but a perfect match is usually not
+a guarantee.  This can have the effect of bringing a system from a state where
+TSC is synchronized back to a state where TSC synchronization flaws, however
+small, may be exposed to the OS and any virtualization environment.
+
+3.3) TSC and multi-socket / NUMA
+
+Multi-socket systems, especially large multi-socket systems are likely to have
+individual clocksources rather than a single, universally distributed clock.
+Since these clocks are driven by different crystals, they will not have
+perfectly matched frequency, and temperature and electrical variations will
+cause the CPU clocks, and thus the TSCs to drift over time.  Depending on the
+exact clock and bus design, the drift may or may not be fixed in absolute
+error, and may accumulate over time.
+
+In addition, very large systems may deliberately slew the clocks of individual
+cores.  This technique, known as spread-spectrum clocking, reduces EMI at the
+clock frequency and harmonics of it, which may be required to pass FCC
+standards for telecommunications and computer equipment.
+
+It is recommended not to trust the TSCs to remain synchronized on NUMA or
+multiple socket systems for these reasons.
+
+3.4) TSC and C-states
+
+C-states, or idling states of the processor, especially C1E and deeper sleep
+states may be problematic for TSC as well.  The TSC may stop advancing in such
+a state, resulting in a TSC which is behind that of other CPUs when execution
+is resumed.  Such CPUs must be detected and flagged by the operating system
+based on CPU and chipset identifications.
+
+The TSC in such a case may be corrected by catching it up to a known external
+clocksource.
+
+3.5) TSC frequency change / P-states
+
+To make things slightly more interesting, some CPUs may change frequency.  They
+may or may not run the TSC at the same rate, and because the frequency change
+may be staggered or slewed, at some points in time, the TSC rate may not be
+known other than falling within a range of values.  In this case, the TSC will
+not be a stable time source, and must be calibrated against a known, stable,
+external clock to be a usable source of time.
+
+Whether the TSC runs at a constant rate or scales with the P-state is model
+dependent and must be determined by inspecting CPUID, chipset or vendor
+specific MSR fields.
+
+In addition, some vendors have known bugs where the P-state is actually
+compensated for properly during normal operation, but when the processor is
+inactive, the P-state may be raised temporarily to service cache misses from
+other processors.  In such cases, the TSC on halted CPUs could advance faster
+than that of non-halted processors.  AMD Turion processors are known to have
+this problem.
+
+3.6) TSC and STPCLK / T-states
+
+External signals given to the processor may also have the effect of stopping
+the TSC.  This is typically done for thermal emergency power control to prevent
+an overheating condition, and typically, there is no way to detect that this
+condition has happened.
+
+3.7) TSC virtualization - VMX
+
+VMX provides conditional trapping of RDTSC, RDMSR, WRMSR and RDTSCP
+instructions, which is enough for full virtualization of TSC in any manner.  In
+addition, VMX allows passing through the host TSC plus an additional TSC_OFFSET
+field specified in the VMCS.  Special instructions must be used to read and
+write the VMCS field.
+
+3.8) TSC virtualization - SVM
+
+SVM provides conditional trapping of RDTSC, RDMSR, WRMSR and RDTSCP
+instructions, which is enough for full virtualization of TSC in any manner.  In
+addition, SVM allows passing through the host TSC plus an additional offset
+field specified in the SVM control block.
+
+3.9) TSC feature bits in Linux
+
+In summary, there is no way to guarantee the TSC remains in perfect
+synchronization unless it is explicitly guaranteed by the architecture.  Even
+if so, the TSCs in multi-sockets or NUMA systems may still run independently
+despite being locally consistent.
+
+The following feature bits are used by Linux to signal various TSC attributes,
+but they can only be taken to be meaningful for UP or single node systems.
+
+X86_FEATURE_TSC 		: The TSC is available in hardware
+X86_FEATURE_RDTSCP		: The RDTSCP instruction is available
+X86_FEATURE_CONSTANT_TSC 	: The TSC rate is unchanged with P-states
+X86_FEATURE_NONSTOP_TSC		: The TSC does not stop in C-states
+X86_FEATURE_TSC_RELIABLE	: TSC sync checks are skipped (VMware)
+
+4) Virtualization Problems
+
+Timekeeping is especially problematic for virtualization because a number of
+challenges arise.  The most obvious problem is that time is now shared between
+the host and, potentially, a number of virtual machines.  Thus the virtual
+operating system does not run with 100% usage of the CPU, despite the fact that
+it may very well make that assumption.  It may expect it to remain true to very
+exacting bounds when interrupt sources are disabled, but in reality only its
+virtual interrupt sources are disabled, and the machine may still be preempted
+at any time.  This causes problems as the passage of real time, the injection
+of machine interrupts and the associated clock sources are no longer completely
+synchronized with real time.
+
+This same problem can occur on native hardware to a degree, as SMM mode may
+steal cycles from the naturally on X86 systems when SMM mode is used by the
+BIOS, but not in such an extreme fashion.  However, the fact that SMM mode may
+cause similar problems to virtualization makes it a good justification for
+solving many of these problems on bare metal.
+
+4.1) Interrupt clocking
+
+One of the most immediate problems that occurs with legacy operating systems
+is that the system timekeeping routines are often designed to keep track of
+time by counting periodic interrupts.  These interrupts may come from the PIT
+or the RTC, but the problem is the same: the host virtualization engine may not
+be able to deliver the proper number of interrupts per second, and so guest
+time may fall behind.  This is especially problematic if a high interrupt rate
+is selected, such as 1000 HZ, which is unfortunately the default for many Linux
+guests.
+
+There are three approaches to solving this problem; first, it may be possible
+to simply ignore it.  Guests which have a separate time source for tracking
+'wall clock' or 'real time' may not need any adjustment of their interrupts to
+maintain proper time.  If this is not sufficient, it may be necessary to inject
+additional interrupts into the guest in order to increase the effective
+interrupt rate.  This approach leads to complications in extreme conditions,
+where host load or guest lag is too much to compensate for, and thus another
+solution to the problem has risen: the guest may need to become aware of lost
+ticks and compensate for them internally.  Although promising in theory, the
+implementation of this policy in Linux has been extremely error prone, and a
+number of buggy variants of lost tick compensation are distributed across
+commonly used Linux systems.
+
+Windows uses periodic RTC clocking as a means of keeping time internally, and
+thus requires interrupt slewing to keep proper time.  It does use a low enough
+rate (ed: is it 18.2 Hz?) however that it has not yet been a problem in
+practice.
+
+4.2) TSC sampling and serialization
+
+As the highest precision time source available, the cycle counter of the CPU
+has aroused much interest from developers.  As explained above, this timer has
+many problems unique to its nature as a local, potentially unstable and
+potentially unsynchronized source.  One issue which is not unique to the TSC,
+but is highlighted because of its very precise nature is sampling delay.  By
+definition, the counter, once read is already old.  However, it is also
+possible for the counter to be read ahead of the actual use of the result.
+This is a consequence of the superscalar execution of the instruction stream,
+which may execute instructions out of order.  Such execution is called
+non-serialized.  Forcing serialized execution is necessary for precise
+measurement with the TSC, and requires a serializing instruction, such as CPUID
+or an MSR read.
+
+Since CPUID may actually be virtualized by a trap and emulate mechanism, this
+serialization can pose a performance issue for hardware virtualization.  An
+accurate time stamp counter reading may therefore not always be available, and
+it may be necessary for an implementation to guard against "backwards" reads of
+the TSC as seen from other CPUs, even in an otherwise perfectly synchronized
+system.
+
+4.3) Timespec aliasing
+
+Additionally, this lack of serialization from the TSC poses another challenge
+when using results of the TSC when measured against another time source.  As
+the TSC is much higher precision, many possible values of the TSC may be read
+while another clock is still expressing the same value.
+
+That is, you may read (T,T+10) while external clock C maintains the same value.
+Due to non-serialized reads, you may actually end up with a range which
+fluctuates - from (T-1.. T+10).  Thus, any time calculated from a TSC, but
+calibrated against an external value may have a range of valid values.
+Re-calibrating this computation may actually cause time, as computed after the
+calibration, to go backwards, compared with time computed before the
+calibration.
+
+This problem is particularly pronounced with an internal time source in Linux,
+the kernel time, which is expressed in the theoretically high resolution
+timespec - but which advances in much larger granularity intervals, sometimes
+at the rate of jiffies, and possibly in catchup modes, at a much larger step.
+
+This aliasing requires care in the computation and recalibration of kvmclock
+and any other values derived from TSC computation (such as TSC virtualization
+itself).
+
+4.4) Migration
+
+Migration of a virtual machine raises problems for timekeeping in two ways.
+First, the migration itself may take time, during which interrupts cannot be
+delivered, and after which, the guest time may need to be caught up.  NTP may
+be able to help to some degree here, as the clock correction required is
+typically small enough to fall in the NTP-correctable window.
+
+An additional concern is that timers based off the TSC (or HPET, if the raw bus
+clock is exposed) may now be running at different rates, requiring compensation
+in some way in the hypervisor by virtualizing these timers.  In addition,
+migrating to a faster machine may preclude the use of a passthrough TSC, as a
+faster clock cannot be made visible to a guest without the potential of time
+advancing faster than usual.  A slower clock is less of a problem, as it can
+always be caught up to the original rate.  KVM clock avoids these problems by
+simply storing multipliers and offsets against the TSC for the guest to convert
+back into nanosecond resolution values.
+
+4.5) Scheduling
+
+Since scheduling may be based on precise timing and firing of interrupts, the
+scheduling algorithms of an operating system may be adversely affected by
+virtualization.  In theory, the effect is random and should be universally
+distributed, but in contrived as well as real scenarios (guest device access,
+causes of virtualization exits, possible context switch), this may not always
+be the case.  The effect of this has not been well studied.
+
+In an attempt to work around this, several implementations have provided a
+paravirtualized scheduler clock, which reveals the true amount of CPU time for
+which a virtual machine has been running.
+
+4.6) Watchdogs
+
+Watchdog timers, such as the lock detector in Linux may fire accidentally when
+running under hardware virtualization due to timer interrupts being delayed or
+misinterpretation of the passage of real time.  Usually, these warnings are
+spurious and can be ignored, but in some circumstances it may be necessary to
+disable such detection.
+
+4.7) Delays and precision timing
+
+Precise timing and delays may not be possible in a virtualized system.  This
+can happen if the system is controlling physical hardware, or issues delays to
+compensate for slower I/O to and from devices.  The first issue is not solvable
+in general for a virtualized system; hardware control software can't be
+adequately virtualized without a full real-time operating system, which would
+require an RT aware virtualization platform.
+
+The second issue may cause performance problems, but this is unlikely to be a
+significant issue.  In many cases these delays may be eliminated through
+configuration or paravirtualization.
+
+4.8) Covert channels and leaks
+
+In addition to the above problems, time information will inevitably leak to the
+guest about the host in anything but a perfect implementation of virtualized
+time.  This may allow the guest to infer the presence of a hypervisor (as in a
+red-pill type detection), and it may allow information to leak between guests
+by using CPU utilization itself as a signalling channel.  Preventing such
+problems would require completely isolated virtual time which may not track
+real time any longer.  This may be useful in certain security or QA contexts,
+but in general isn't recommended for real-world deployment scenarios.