3.16.70-rc1 review patch. If anyone has any objections, please let me know.
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From: Sean Christopherson sean.j.christopherson@intel.com
commit ddfd1730fd829743e41213e32ccc8b4aa6dc8325 upstream.
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 installed.
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 documentation.
Fixes: 56f17dd3fbc4 ("kvm: x86: fix stale mmio cache bug") Signed-off-by: Sean Christopherson sean.j.christopherson@intel.com Signed-off-by: Paolo Bonzini pbonzini@redhat.com [ Signed-off-by: Ben Hutchings ben@decadent.org.uk --- arch/x86/kvm/x86.h | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-)
--- a/arch/x86/kvm/x86.h +++ b/arch/x86/kvm/x86.h @@ -75,10 +75,15 @@ static inline u32 bit(int bitno) 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; + vcpu->arch.mmio_gva = 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)