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Diffstat (limited to 'Documentation/virtual/kvm/mmu.txt')
| -rw-r--r-- | Documentation/virtual/kvm/mmu.txt | 91 |
1 files changed, 83 insertions, 8 deletions
diff --git a/Documentation/virtual/kvm/mmu.txt b/Documentation/virtual/kvm/mmu.txt index 43fcb761ed1..29089417614 100644 --- a/Documentation/virtual/kvm/mmu.txt +++ b/Documentation/virtual/kvm/mmu.txt @@ -191,12 +191,12 @@ Shadow pages contain the following information: 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 @@ -210,6 +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 =========== @@ -258,14 +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 @@ -351,6 +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 =============== |