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authorPeter Xu <peterx@redhat.com>2020-03-05 18:57:08 +0300
committerPaolo Bonzini <pbonzini@redhat.com>2020-03-16 19:58:56 +0300
commit3ac40c404c60f20dd07920810b1195125f193e1e (patch)
tree22544299f4b5cfd643a838641be49ab2d774f134 /Documentation/virt/kvm/locking.rst
parent78f2145c4d937a4770f365d6f5dc6eb825658d10 (diff)
downloadlinux-3ac40c404c60f20dd07920810b1195125f193e1e.tar.xz
KVM: Documentation: Update fast page fault for indirect sp
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>
Diffstat (limited to 'Documentation/virt/kvm/locking.rst')
-rw-r--r--Documentation/virt/kvm/locking.rst11
1 files changed, 5 insertions, 6 deletions
diff --git a/Documentation/virt/kvm/locking.rst b/Documentation/virt/kvm/locking.rst
index c02291beac3f..b21a34c34a21 100644
--- a/Documentation/virt/kvm/locking.rst
+++ b/Documentation/virt/kvm/locking.rst
@@ -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