KVM: document memory barriers for kvm->vcpus/kvm->online_vcpus

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index bd1097a..81089cf 100644 (file)
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -427,6 +427,10 @@ struct kvm {
 
 static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i)
 {
+       /* Pairs with smp_wmb() in kvm_vm_ioctl_create_vcpu, in case
+        * the caller has read kvm->online_vcpus before (as is the case
+        * for kvm_for_each_vcpu, for example).
+        */
        smp_rmb();
        return kvm->vcpus[i];
 }

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 8dc4828..d8db2f8 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2206,6 +2206,11 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id)
        }
 
        kvm->vcpus[atomic_read(&kvm->online_vcpus)] = vcpu;
+
+       /*
+        * Pairs with smp_rmb() in kvm_get_vcpu.  Write kvm->vcpus
+        * before kvm->online_vcpu's incremented value.
+        */
        smp_wmb();
        atomic_inc(&kvm->online_vcpus);