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Diffstat (limited to 'Documentation/virtual/kvm/mmu.txt')
| -rw-r--r-- | Documentation/virtual/kvm/mmu.txt | 98 |
1 files changed, 83 insertions, 15 deletions
diff --git a/Documentation/virtual/kvm/mmu.txt b/Documentation/virtual/kvm/mmu.txt index fa5f1dbc6b2..29089417614 100644 --- a/Documentation/virtual/kvm/mmu.txt +++ b/Documentation/virtual/kvm/mmu.txt @@ -187,23 +187,16 @@ Shadow pages contain the following information: 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. + 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 @@ -217,6 +210,24 @@ Shadow pages contain the following information: 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 =========== @@ -265,14 +276,26 @@ This is the most complicated event. The cause of a page fault can be: 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; call the emulator - to emulate the instruction instead + - 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 @@ -358,6 +381,51 @@ 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. + + Further reading =============== |