From cde9af6e79046e12cd08d161139b1d5e57e9fbac Mon Sep 17 00:00:00 2001 From: Andrew Jones Date: Wed, 26 Apr 2017 22:32:24 +0200 Subject: KVM: add explicit barrier to kvm_vcpu_kick kvm_vcpu_kick() must issue a general memory barrier prior to reading vcpu->mode in order to ensure correctness of the mutual-exclusion memory barrier pattern used with vcpu->requests. While the cmpxchg called from kvm_vcpu_kick(): kvm_vcpu_kick kvm_arch_vcpu_should_kick kvm_vcpu_exiting_guest_mode cmpxchg implies general memory barriers before and after the operation, that implication is only valid when cmpxchg succeeds. We need an explicit barrier for when it fails, otherwise a VCPU thread on its entry path that reads zero for vcpu->requests does not exclude the possibility the requesting thread sees !IN_GUEST_MODE when it reads vcpu->mode. kvm_make_all_cpus_request already had a barrier, so we remove it, as now it would be redundant. Signed-off-by: Andrew Jones Signed-off-by: Paolo Bonzini --- include/linux/kvm_host.h | 6 ++++++ 1 file changed, 6 insertions(+) (limited to 'include/linux/kvm_host.h') diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index a805ddcb7eb0..84c5396564f7 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -270,6 +270,12 @@ struct kvm_vcpu { static inline int kvm_vcpu_exiting_guest_mode(struct kvm_vcpu *vcpu) { + /* + * The memory barrier ensures a previous write to vcpu->requests cannot + * be reordered with the read of vcpu->mode. It pairs with the general + * memory barrier following the write of vcpu->mode in VCPU RUN. + */ + smp_mb__before_atomic(); return cmpxchg(&vcpu->mode, IN_GUEST_MODE, EXITING_GUEST_MODE); } -- cgit v1.2.3