[PATCH 0/2] KVM: x86/mmu: Use LOCK CMPXCHG to clear Accessed bits

[Sean Christopherson <seanjc@google.com>]

Use LOCK CMPXCHG instead of LOCK AND or LOCK BTR to clear Accessed bits when
aging SPTEs, as LOCK AND could clobber a !PRESENT SPTE.  Specificaly, as
flagged by AI of some kind, a LOCK AND in the TDP MMU could clobber a
FROZEN_SPTE and ultimately result in a false negative since the Accessed bit
is included in FROZEN_SPTE.

I initially was planning on tweaking the FROZEN_SPTE value to make it ok to
use LOCK AND on Accessed bits, but AFAICT there's no meaningful downside to
using CMPXCHG.

Sean Christopherson (2):
  KVM: x86/mmu: Use CMPXCHG when clearing Accessed bit in TDP MMU
  KVM: x86/mmu: Use CMPXCHG when clearing Accessed bit in the shadow MMU

 arch/x86/kvm/mmu/mmu.c      | 27 ++++++++++++++-------------
 arch/x86/kvm/mmu/tdp_iter.h |  7 +++++++
 arch/x86/kvm/mmu/tdp_mmu.c  | 20 +++++++++-----------
 3 files changed, 30 insertions(+), 24 deletions(-)


base-commit: 271255273d5ff348fe29d89fe4712b2f7f7907c3
-- 
2.55.0.229.g6434b31f56-goog