Revert "vhost: cache used event for better performance"

This reverts commit 809ecb9b. Since it
was reported to break vhost_net. We want to cache used event and use
it to check for notification. The assumption was that guest won't move
the event idx back, but this could happen in fact when 16 bit index
wraps around after 64K entries.
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/vhost/vhost.c

@@ -308,7 +308,6 @@ static void vhost_vq_reset(struct vhost_dev *dev,
 	vq->avail = NULL;
 	vq->used = NULL;
 	vq->last_avail_idx = 0;
-	vq->last_used_event = 0;
 	vq->avail_idx = 0;
 	vq->last_used_idx = 0;
 	vq->signalled_used = 0;
@@ -1402,7 +1401,7 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
 			r = -EINVAL;
 			break;
 		}
-		vq->last_avail_idx = vq->last_used_event = s.num;
+		vq->last_avail_idx = s.num;
 		/* Forget the cached index value. */
 		vq->avail_idx = vq->last_avail_idx;
 		break;
@@ -2241,6 +2240,10 @@ static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
 	__u16 old, new;
 	__virtio16 event;
 	bool v;
+	/* Flush out used index updates. This is paired
+	 * with the barrier that the Guest executes when enabling
+	 * interrupts. */
+	smp_mb();
 
 	if (vhost_has_feature(vq, VIRTIO_F_NOTIFY_ON_EMPTY) &&
 	    unlikely(vq->avail_idx == vq->last_avail_idx))
@@ -2248,10 +2251,6 @@ static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
 
 	if (!vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) {
 		__virtio16 flags;
-		/* Flush out used index updates. This is paired
-		 * with the barrier that the Guest executes when enabling
-		 * interrupts. */
-		smp_mb();
 		if (vhost_get_avail(vq, flags, &vq->avail->flags)) {
 			vq_err(vq, "Failed to get flags");
 			return true;
@@ -2266,26 +2265,11 @@ static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
 	if (unlikely(!v))
 		return true;
 
-	/* We're sure if the following conditions are met, there's no
-	 * need to notify guest:
-	 * 1) cached used event is ahead of new
-	 * 2) old to new updating does not cross cached used event. */
-	if (vring_need_event(vq->last_used_event, new + vq->num, new) &&
-	    !vring_need_event(vq->last_used_event, new, old))
-		return false;
-
-	/* Flush out used index updates. This is paired
-	 * with the barrier that the Guest executes when enabling
-	 * interrupts. */
-	smp_mb();
-
 	if (vhost_get_avail(vq, event, vhost_used_event(vq))) {
 		vq_err(vq, "Failed to get used event idx");
 		return true;
 	}
-	vq->last_used_event = vhost16_to_cpu(vq, event);
-
-	return vring_need_event(vq->last_used_event, new, old);
+	return vring_need_event(vhost16_to_cpu(vq, event), new, old);
 }
 
 /* This actually signals the guest, using eventfd. */

drivers/vhost/vhost.h

@@ -115,9 +115,6 @@ struct vhost_virtqueue {
 	/* Last index we used. */
 	u16 last_used_idx;
 
-	/* Last used evet we've seen */
-	u16 last_used_event;
-
 	/* Used flags */
 	u16 used_flags;