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>

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;

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);