virtual/kvm/mmu.txt

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.
  slot_bitmap:
    A bitmap containing one bit per memory slot.  If the page contains a pte
    mapping a page from memory slot n, then bit n of slot_bitmap will be set
    (if a page is aliased among several slots, then it is not guaranteed that
    all slots will be marked).
    Used during dirty logging to avoid scanning a shadow page if none if its
    pages need tracking.
  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.
  multimapped:
    Whether there exist multiple sptes pointing at this page.
  parent_pte/parent_ptes:
    If multimapped is zero, parent_pte points at the single spte that points at
    this page's spt.  Otherwise, parent_ptes 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.

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 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; call the emulator
     to emulate the instruction instead
 - walk the shadow page table to find the spte for the translation,
   instantiating missing intermediate page tables as necessary
 - 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.

Further reading
===============

- NPT presentation from KVM Forum 2008
  http://www.linux-kvm.org/wiki/images/c/c8/KvmForum2008%24kdf2008_21.pdf