Patchwork [v2,03/27] KVM: x86/mmu: Do not cache MMIO accesses while memslots are in flux

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Submitter Christopherson, Sean J
Date Feb. 5, 2019, 9:01 p.m.
Message ID <>
Download mbox | patch
Permalink /patch/719003/
State New
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Christopherson, Sean J - Feb. 5, 2019, 9:01 p.m.
When installing new memslots, KVM sets bit 0 of the generation number to
indicate that an update is in-progress.  Until the update is complete,
there are no guarantees as to whether a vCPU will see the old or the new
memslots.  Explicity prevent caching MMIO accesses so as to avoid using
an access cached from the old memslots after the new memslots have been

Note that it is unclear whether or not disabling caching during the
update window is strictly necessary as there is no definitive
documentation as to what ordering guarantees KVM provides with respect
to updating memslots.  That being said, the MMIO spte code does not
allow reusing sptes created while an update is in-progress, and the
associated documentation explicitly states:

    We do not want to use an MMIO sptes created with an odd generation
    number, ...  If KVM is unlucky and creates an MMIO spte while the
    low bit is 1, the next access to the spte will always be a cache miss.

At the very least, disabling the per-vCPU MMIO cache during updates will
make its behavior consistent with the MMIO spte behavior and

Fixes: 56f17dd3fbc4 ("kvm: x86: fix stale mmio cache bug")
Cc: <>
Signed-off-by: Sean Christopherson <>
 arch/x86/kvm/x86.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)


diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index 224cd0a47568..20ede17202bf 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -181,6 +181,11 @@  static inline bool emul_is_noncanonical_address(u64 la,
 static inline void vcpu_cache_mmio_info(struct kvm_vcpu *vcpu,
 					gva_t gva, gfn_t gfn, unsigned access)
+	u64 gen = kvm_memslots(vcpu->kvm)->generation;
+	if (unlikely(gen & 1))
+		return;
 	 * If this is a shadow nested page table, the "GVA" is
 	 * actually a nGPA.
@@ -188,7 +193,7 @@  static inline void vcpu_cache_mmio_info(struct kvm_vcpu *vcpu,
 	vcpu->arch.mmio_gva = mmu_is_nested(vcpu) ? 0 : gva & PAGE_MASK;
 	vcpu->arch.access = access;
 	vcpu->arch.mmio_gfn = gfn;
-	vcpu->arch.mmio_gen = kvm_memslots(vcpu->kvm)->generation;
+	vcpu->arch.mmio_gen = gen;
 static inline bool vcpu_match_mmio_gen(struct kvm_vcpu *vcpu)