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Commit 5a9b3830 authored by Xiao Guangrong's avatar Xiao Guangrong Committed by Gleb Natapov
Browse files

KVM: MMU: document fast invalidate all mmio sptes 



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

Signed-off-by: default avatarXiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent f6f8adee

Documentation/virtual/kvm/mmu.txt

+26 −0
Original line number Original line Diff line number Diff line
@@ -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

 - 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.

   accessing MMIO and cached MMIO information is available.

   - walk shadow page table

   - 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

   - cache the information to vcpu->arch.mmio_gva, vcpu->arch.access and

     vcpu->arch.mmio_gfn, and call the emulator

     vcpu->arch.mmio_gfn, and call the emulator

 - If both P bit and R/W bit of error code are set, this could possibly

 - 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

operation needs to take the MMU lock, the lock can be released periodically

so that the VCPUs can make progress.

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

Further reading

===============

===============