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KVM: Documentation: Update fast page fault for indirect sp

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Clarify locking.rst to mention early that we're not enabling fast page
fault for indirect sps.  The previous wording is confusing, in that it
seems the proposed solution has been already implemented but it has not.

Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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Peter Xu 2020-03-05 10:57:08 -05:00 committed by Paolo Bonzini
parent 78f2145c4d
commit 3ac40c404c
1 changed files with 5 additions and 6 deletions
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Documentation/virt/kvm/locking.rst
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@ -96,19 +96,18 @@ will happen:
We dirty-log for gfn1, that means gfn2 is lost in dirty-bitmap.
For direct sp, we can easily avoid it since the spte of direct sp is fixed
to gfn. For indirect sp, before we do cmpxchg, we call gfn_to_pfn_atomic()
to pin gfn to pfn, because after gfn_to_pfn_atomic():
to gfn. For indirect sp, we disabled fast page fault for simplicity.
A solution for indirect sp could be to pin the gfn, for example via
kvm_vcpu_gfn_to_pfn_atomic, before the cmpxchg. After the pinning:
- We have held the refcount of pfn that means the pfn can not be freed and
be reused for another gfn.
- The pfn is writable that means it can not be shared between different gfns
- The pfn is writable and therefore it cannot be shared between different gfns
by KSM.
Then, we can ensure the dirty bitmaps is correctly set for a gfn.
Currently, to simplify the whole things, we disable fast page fault for
indirect shadow page.
2) Dirty bit tracking
In the origin code, the spte can be fast updated (non-atomically) if the