Merge: virtio_ring: Fix error reporting in virtqueue_resize

MR: https://gitlab.com/redhat/centos-stream/src/kernel/centos-stream-10/-/merge_requests/1170

JIRA: https://issues.redhat.com/browse/RHEL-84409

Enforce minimum TX ring size for reliability

The `tx_may_stop()` logic stops TX queues if free descriptors
(`sq->vq->num_free`) fall below the threshold of (`MAX_SKB_FRAGS` + 2).
If the total ring size (`ring_num`) is not strictly greater than this
value, queues can become persistently stopped or stop after minimal
use, severely degrading performance.

A single sk_buff transmission typically requires descriptors for:
- The virtio_net_hdr (1 descriptor)
- The sk_buff's linear data (head) (1 descriptor)
- Paged fragments (up to MAX_SKB_FRAGS descriptors)

Signed-off-by: Laurent Vivier <lvivier@redhat.com>

Approved-by: Eugenio Pérez <eperezma@redhat.com>
Approved-by: Cindy Lu <lulu@redhat.com>
Approved-by: CKI KWF Bot <cki-ci-bot+kwf-gitlab-com@redhat.com>

Merged-by: Julio Faracco <jfaracco@redhat.com>

Author: jcfaracco

drivers/net/virtio_net.c

@@ -1065,7 +1065,7 @@ static void check_sq_full_and_disable(struct virtnet_info *vi,
 	 * Since most packets only take 1 or 2 ring slots, stopping the queue
 	 * early means 16 slots are typically wasted.
 	 */
-	if (sq->vq->num_free < 2+MAX_SKB_FRAGS) {
+	if (sq->vq->num_free < MAX_SKB_FRAGS + 2) {
 		struct netdev_queue *txq = netdev_get_tx_queue(dev, qnum);
 
 		netif_tx_stop_queue(txq);
@@ -1078,7 +1078,7 @@ static void check_sq_full_and_disable(struct virtnet_info *vi,
 		} else if (unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
 			/* More just got used, free them then recheck. */
 			free_old_xmit(sq, txq, false);
-			if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) {
+			if (sq->vq->num_free >= MAX_SKB_FRAGS + 2) {
 				netif_start_subqueue(dev, qnum);
 				u64_stats_update_begin(&sq->stats.syncp);
 				u64_stats_inc(&sq->stats.wake);
@@ -2797,7 +2797,7 @@ static void virtnet_poll_cleantx(struct receive_queue *rq, int budget)
 			free_old_xmit(sq, txq, !!budget);
 		} while (unlikely(!virtqueue_enable_cb_delayed(sq->vq)));
 
-		if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS) {
+		if (sq->vq->num_free >= MAX_SKB_FRAGS + 2) {
 			if (netif_tx_queue_stopped(txq)) {
 				u64_stats_update_begin(&sq->stats.syncp);
 				u64_stats_inc(&sq->stats.wake);
@@ -2990,7 +2990,7 @@ static int virtnet_poll_tx(struct napi_struct *napi, int budget)
 	virtqueue_disable_cb(sq->vq);
 	free_old_xmit(sq, txq, !!budget);
 
-	if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS) {
+	if (sq->vq->num_free >= MAX_SKB_FRAGS + 2) {
 		if (netif_tx_queue_stopped(txq)) {
 			u64_stats_update_begin(&sq->stats.syncp);
 			u64_stats_inc(&sq->stats.wake);
@@ -3219,6 +3219,12 @@ static int virtnet_tx_resize(struct virtnet_info *vi, struct send_queue *sq,
 {
 	int qindex, err;
 
+	if (ring_num <= MAX_SKB_FRAGS + 2) {
+		netdev_err(vi->dev, "tx size (%d) cannot be smaller than %d\n",
+			   ring_num, MAX_SKB_FRAGS + 2);
+		return -EINVAL;
+	}
+
 	qindex = sq - vi->sq;
 
 	virtnet_tx_pause(vi, sq);

drivers/virtio/virtio_ring.c

@@ -2741,7 +2741,7 @@ int virtqueue_resize(struct virtqueue *_vq, u32 num,
 		     void (*recycle_done)(struct virtqueue *vq))
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
-	int err;
+	int err, err_reset;
 
 	if (num > vq->vq.num_max)
 		return -E2BIG;
@@ -2763,7 +2763,11 @@ int virtqueue_resize(struct virtqueue *_vq, u32 num,
 	else
 		err = virtqueue_resize_split(_vq, num);
 
-	return virtqueue_enable_after_reset(_vq);
+	err_reset = virtqueue_enable_after_reset(_vq);
+	if (err_reset)
+		return err_reset;
+
+	return err;
 }
 EXPORT_SYMBOL_GPL(virtqueue_resize);