KVM: Document mmu

Signed-off-by: Avi Kivity <avi@redhat.com>

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+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)
+- reliablity:  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 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 gva, but not
+vice versa.
+
+These gvas 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 an 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.cr4_nxe:
+    Contains the value of efer.nxe for which the page is valid.
+  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 containig 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.
+  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 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
+
+Further reading
+===============
+
+- NPT presentation from KVM Forum 2008
+  http://www.linux-kvm.org/wiki/images/c/c8/KvmForum2008%24kdf2008_21.pdf
+