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KVM: MMU: document fast invalidate all mmio sptes

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Document it to Documentation/virtual/kvm/mmu.txt

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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Xiao Guangrong 2013-06-19 17:09:25 +08:00 committed by Gleb Natapov
parent f6f8adeef5
commit 5a9b3830d4
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Documentation/virtual/kvm/mmu.txt
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@ -279,6 +279,8 @@ 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
@ -400,6 +402,30 @@ 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
===============