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diff --git a/Documentation/virtual/kvm/mmu.txt b/Documentation/virtual/kvm/mmu.txt deleted file mode 100644 index 2efe0efc516e..000000000000 --- a/Documentation/virtual/kvm/mmu.txt +++ /dev/null @@ -1,449 +0,0 @@ -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. Emulated hardware also -able to expose 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.gpte_is_8_bytes=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.gpte_is_8_bytes: - Reflects the size of the guest PTE for which the page is valid, i.e. '1' - if 64-bit gptes are in use, '0' if 32-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). - role.smap_andnot_wp: - Contains the value of cr4.smap && !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). - role.ept_sp: - This is a virtual flag to denote a shadowed nested EPT page. ept_sp - is true if "cr0_wp && smap_andnot_wp", an otherwise invalid combination. - role.smm: - Is 1 if the page is valid in system management mode. This field - determines which of the kvm_memslots array was used to build this - shadow page; it is also used to go back from a struct kvm_mmu_page - to a memslot, through the kvm_memslots_for_spte_role macro and - __gfn_to_memslot. - role.ad_disabled: - Is 1 if the MMU instance cannot use A/D bits. EPT did not have A/D - bits before Haswell; shadow EPT page tables also cannot use A/D bits - if the L1 hypervisor does not enable them. - 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 page 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 sptes. - 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. - 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 are two additional complications: -- 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. For this to work, KVM forces EFER.NX to 1 when - shadow paging is in use. -- if CR4.SMAP is disabled: since the page has been changed to a kernel - page, it can not be reused when CR4.SMAP is enabled. We set - CR4.SMAP && !CR0.WP into shadow page's role to avoid this case. Note, - here we do not care the case that CR4.SMAP is enabled since KVM will - directly inject #PF to guest due to failed permission check. - -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 ->disallow_lpage set of -arrays for each memory slot and large page size. Every write protected page -causes its disallow_lpage to be incremented, thus preventing instantiation of -a large spte. The frames at the end of an unaligned memory slot have -artificially inflated ->disallow_lpages so they can never be instantiated. - -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. - -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, bit 63 of kvm_memslots(kvm)->generation set to 1 only 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 "update in-progress" bit of the -generation is not stored in MMIO spte, and is so is implicitly zero when the -generation is extracted out of the spte. If KVM is unlucky and creates an MMIO -spte while an update is in-progress, the next access to the spte will always be -a cache miss. For example, a subsequent access during the update window will -miss due to the in-progress flag diverging, while an access after the update -window closes will have a higher generation number (as compared to the spte). - - -Further reading -=============== - -- NPT presentation from KVM Forum 2008 - http://www.linux-kvm.org/images/c/c8/KvmForum2008%24kdf2008_21.pdf - |