From: Paolo Bonzini <pbonzini@redhat.com>
Date: Tue, 3 May 2022 11:23:08 +0000 (-0400)
Subject: Merge branch 'kvm-tdp-mmu-atomicity-fix' into HEAD
X-Git-Url: http://git.maquefel.me/?a=commitdiff_plain;h=6ea6581f127128a07e18f61ea691a22741a3c714;p=linux.git

Merge branch 'kvm-tdp-mmu-atomicity-fix' into HEAD

We are dropping A/D bits (and W bits) in the TDP MMU.  Even if mmu_lock
is held for write, as volatile SPTEs can be written by other tasks/vCPUs
outside of mmu_lock.

Attempting to prove that bug exposed another notable goof, which has been
lurking for a decade, give or take: KVM treats _all_ MMU-writable SPTEs
as volatile, even though KVM never clears WRITABLE outside of MMU lock.
As a result, the legacy MMU (and the TDP MMU if not fixed) uses XCHG to
update writable SPTEs.

The fix does not seem to have an easily-measurable affect on performance;
page faults are so slow that wasting even a few hundred cycles is dwarfed
by the base cost.
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6ea6581f127128a07e18f61ea691a22741a3c714