KVM: Add barriers to allow mmu_notifier_retry to be used locklessly

[platform/adaptation/renesas_rcar/renesas_kernel.git] / include / linux / kvm_host.h

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index d4d4d70..eada8e6 100644 (file)
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -702,12 +702,16 @@ static inline int mmu_notifier_retry(struct kvm_vcpu *vcpu, unsigned long mmu_se
        if (unlikely(vcpu->kvm->mmu_notifier_count))
                return 1;
        /*
-        * Both reads happen under the mmu_lock and both values are
-        * modified under mmu_lock, so there's no need of smb_rmb()
-        * here in between, otherwise mmu_notifier_count should be
-        * read before mmu_notifier_seq, see
-        * mmu_notifier_invalidate_range_end write side.
+        * Ensure the read of mmu_notifier_count happens before the read
+        * of mmu_notifier_seq.  This interacts with the smp_wmb() in
+        * mmu_notifier_invalidate_range_end to make sure that the caller
+        * either sees the old (non-zero) value of mmu_notifier_count or
+        * the new (incremented) value of mmu_notifier_seq.
+        * PowerPC Book3s HV KVM calls this under a per-page lock
+        * rather than under kvm->mmu_lock, for scalability, so
+        * can't rely on kvm->mmu_lock to keep things ordered.
         */
+       smp_rmb();
        if (vcpu->kvm->mmu_notifier_seq != mmu_seq)
                return 1;
        return 0;