Re: [PATCH] KVM: Document mmu

[Gui Jianfeng <guijianfeng@cn.fujitsu.com>]

Avi Kivity wrote:
> Signed-off-by: Avi Kivity <avi@redhat.com>
> ---
>  Documentation/kvm/mmu.txt |  296 +++++++++++++++++++++++++++++++++++++++++++++
>  1 files changed, 296 insertions(+), 0 deletions(-)
>  create mode 100644 Documentation/kvm/mmu.txt
> 
> diff --git a/Documentation/kvm/mmu.txt b/Documentation/kvm/mmu.txt
> new file mode 100644
> index 0000000..176f834
> --- /dev/null
> +++ b/Documentation/kvm/mmu.txt
> @@ -0,0 +1,296 @@
> +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 page 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 virtual 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->gva); note that two gpas may alias to the same gva, but not

Do you mean (gpa->hva)?

> +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); one 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
> +

-- 
Regards
Gui Jianfeng