[PATCH v3 3/3] vhost: Improve vhost_get_avail_idx() with smp_rmb()

[Gavin Shan <gshan@redhat.com>]

All the callers of vhost_get_avail_idx() are concerned to the memory
barrier, imposed by smp_rmb() to ensure the order of the available
ring entry read and avail_idx read.

Improve vhost_get_avail_idx() so that smp_rmb() is executed when
the avail_idx is advanced. With it, the callers needn't to worry
about the memory barrier.

Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Gavin Shan <gshan@redhat.com>
---
 drivers/vhost/vhost.c | 75 +++++++++++++++----------------------------
 1 file changed, 26 insertions(+), 49 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 32686c79c41d..e6882f4f6ce2 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -1290,10 +1290,28 @@ static void vhost_dev_unlock_vqs(struct vhost_dev *d)
 		mutex_unlock(&d->vqs[i]->mutex);
 }
 
-static inline int vhost_get_avail_idx(struct vhost_virtqueue *vq,
-				      __virtio16 *idx)
+static inline int vhost_get_avail_idx(struct vhost_virtqueue *vq)
 {
-	return vhost_get_avail(vq, *idx, &vq->avail->idx);
+	__virtio16 avail_idx;
+	int r;
+
+	r = vhost_get_avail(vq, avail_idx, &vq->avail->idx);
+	if (unlikely(r)) {
+		vq_err(vq, "Failed to access avail idx at %p\n",
+		       &vq->avail->idx);
+		return r;
+	}
+
+	vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
+	if (vq->avail_idx != vq->last_avail_idx) {
+		/* Ensure the available ring entry read happens
+		 * before the avail_idx read when the avail_idx
+		 * is advanced.
+		 */
+		smp_rmb();
+	}
+
+	return 0;
 }
 
 static inline int vhost_get_avail_head(struct vhost_virtqueue *vq,
@@ -2499,7 +2517,6 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
 	struct vring_desc desc;
 	unsigned int i, head, found = 0;
 	u16 last_avail_idx;
-	__virtio16 avail_idx;
 	__virtio16 ring_head;
 	int ret, access;
 
@@ -2507,12 +2524,8 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
 	last_avail_idx = vq->last_avail_idx;
 
 	if (vq->avail_idx == vq->last_avail_idx) {
-		if (unlikely(vhost_get_avail_idx(vq, &avail_idx))) {
-			vq_err(vq, "Failed to access avail idx at %p\n",
-				&vq->avail->idx);
+		if (unlikely(vhost_get_avail_idx(vq)))
 			return -EFAULT;
-		}
-		vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
 
 		if (unlikely((u16)(vq->avail_idx - last_avail_idx) > vq->num)) {
 			vq_err(vq, "Guest moved used index from %u to %u",
@@ -2525,11 +2538,6 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
 		 */
 		if (vq->avail_idx == last_avail_idx)
 			return vq->num;
-
-		/* Only get avail ring entries after they have been
-		 * exposed by guest.
-		 */
-		smp_rmb();
 	}
 
 	/* Grab the next descriptor number they're advertising, and increment
@@ -2790,35 +2798,19 @@ EXPORT_SYMBOL_GPL(vhost_add_used_and_signal_n);
 /* return true if we're sure that avaiable ring is empty */
 bool vhost_vq_avail_empty(struct vhost_dev *dev, struct vhost_virtqueue *vq)
 {
-	__virtio16 avail_idx;
-	int r;
-
 	if (vq->avail_idx != vq->last_avail_idx)
 		return false;
 
-	r = vhost_get_avail_idx(vq, &avail_idx);
-	if (unlikely(r))
-		return false;
-
-	vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
-	if (vq->avail_idx != vq->last_avail_idx) {
-		/* Since we have updated avail_idx, the following
-		 * call to vhost_get_vq_desc() will read available
-		 * ring entries. Make sure that read happens after
-		 * the avail_idx read.
-		 */
-		smp_rmb();
+	if (unlikely(vhost_get_avail_idx(vq)))
 		return false;
-	}
 
-	return true;
+	return vq->avail_idx == vq->last_avail_idx;
 }
 EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);
 
 /* OK, now we need to know about added descriptors. */
 bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
 {
-	__virtio16 avail_idx;
 	int r;
 
 	if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY))
@@ -2842,25 +2834,10 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
 	/* They could have slipped one in as we were doing that: make
 	 * sure it's written, then check again. */
 	smp_mb();
-	r = vhost_get_avail_idx(vq, &avail_idx);
-	if (r) {
-		vq_err(vq, "Failed to check avail idx at %p: %d\n",
-		       &vq->avail->idx, r);
+	if (unlikely(vhost_get_avail_idx(vq)))
 		return false;
-	}
-
-	vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
-	if (vq->avail_idx != vq->last_avail_idx) {
-		/* Since we have updated avail_idx, the following
-		 * call to vhost_get_vq_desc() will read available
-		 * ring entries. Make sure that read happens after
-		 * the avail_idx read.
-		 */
-		smp_rmb();
-		return true;
-	}
 
-	return false;
+	return vq->avail_idx != vq->last_avail_idx;
 }
 EXPORT_SYMBOL_GPL(vhost_enable_notify);
 
-- 
2.44.0