KVM: Add barriers to allow mmu_notifier_retry to be used locklessly

This adds an smp_wmb in kvm_mmu_notifier_invalidate_range_end() and an
smp_rmb in mmu_notifier_retry() so that mmu_notifier_retry() will give
the correct answer when called without kvm->mmu_lock being held.
PowerPC Book3S HV KVM wants to use a bitlock per guest page rather than
a single global spinlock in order to improve the scalability of updates
to the guest MMU hashed page table, and so needs this.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Acked-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>

2 files changed, 12 insertions, 8 deletions

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index d4d4d7092110..eada8e69fe58 100644
--- 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;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 64be836f3348..9f32bffd37c0 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -357,11 +357,11 @@ static void kvm_mmu_notifier_invalidate_range_end(struct mmu_notifier *mn,
          * been freed.
          */
         kvm->mmu_notifier_seq++;
+        smp_wmb();
         /*
          * The above sequence increase must be visible before the
-         * below count decrease but both values are read by the kvm
-         * page fault under mmu_lock spinlock so we don't need to add
-         * a smb_wmb() here in between the two.
+         * below count decrease, which is ensured by the smp_wmb above
+         * in conjunction with the smp_rmb in mmu_notifier_retry().
          */
         kvm->mmu_notifier_count--;
         spin_unlock(&kvm->mmu_lock);