[PATCH net-next v2 2/2] virtio-net: xsk: support tx wake up

[menglong8 . dong]

From: Menglong Dong <dongml2@xxxxxxxxxxxxxxx>

For now, XDP_RING_NEED_WAKEUP is not supported properly by the virtio-net
in the tx path for example: we set xsk_set_tx_need_wakeup() in
virtnet_xsk_xmit(), but we didn't call xsk_clear_tx_need_wakeup()
anywhere, which means the user will call send() for every packet.

We call xsk_set_tx_need_wakeup() after virtnet_xsk_xmit_batch() if sq->vq
is empty, as we can't be wakeup by the skb_xmit_done() in this case.
Otherwise, we will clear the wakeup flag.

Race condition is considered for tx path.

Fixes: 89f86675cb03 ("virtio_net: xsk: tx: support xmit xsk buffer")
Signed-off-by: Menglong Dong <dongml2@xxxxxxxxxxxxxxx>
---
 drivers/net/virtio_net.c | 27 +++++++++++++++++++++++----
 1 file changed, 23 insertions(+), 4 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 4b5b3fa62008..86b5c1ca568c 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -1459,8 +1459,9 @@ static bool virtnet_xsk_xmit(struct send_queue *sq, struct xsk_buff_pool *pool,
 	struct virtnet_info *vi = sq->vq->vdev->priv;
 	struct virtnet_sq_free_stats stats = {};
 	struct net_device *dev = vi->dev;
+	int sent, vring_size;
+	bool need_wakeup;
 	u64 kicks = 0;
-	int sent;
 
 	/* Avoid to wakeup napi meanless, so call __free_old_xmit instead of
 	 * free_old_xmit().
@@ -1470,8 +1471,29 @@ static bool virtnet_xsk_xmit(struct send_queue *sq, struct xsk_buff_pool *pool,
 	if (stats.xsk)
 		xsk_tx_completed(sq->xsk_pool, stats.xsk);
 
+	vring_size = virtqueue_get_vring_size(sq->vq);
+	need_wakeup = xsk_uses_need_wakeup(pool);
+	/* If the sq->vq is empty, and the tx ring is empty, and the user
+	 * submit an entry to the tx ring after virtnet_xsk_xmit_batch() and
+	 * before xsk_set_tx_need_wakeup(), we will lose the chance to wake
+	 * up the tx napi, so we have to set the need_wakeup flag here.
+	 */
+	if (need_wakeup && vring_size == sq->vq->num_free)
+		xsk_set_tx_need_wakeup(pool);
+
 	sent = virtnet_xsk_xmit_batch(sq, pool, budget, &kicks);
 
+	if (need_wakeup) {
+		if (vring_size == sq->vq->num_free)
+			/* we can't wake up by ourself, and it should be done
+			 * by the user.
+			 */
+			xsk_set_tx_need_wakeup(pool);
+		else
+			/* we can wake up from skb_xmit_done() */
+			xsk_clear_tx_need_wakeup(pool);
+	}
+
 	if (!is_xdp_raw_buffer_queue(vi, sq - vi->sq))
 		check_sq_full_and_disable(vi, vi->dev, sq);
 
@@ -1489,9 +1511,6 @@ static bool virtnet_xsk_xmit(struct send_queue *sq, struct xsk_buff_pool *pool,
 	u64_stats_add(&sq->stats.xdp_tx,  sent);
 	u64_stats_update_end(&sq->stats.syncp);
 
-	if (xsk_uses_need_wakeup(pool))
-		xsk_set_tx_need_wakeup(pool);
-
 	return sent;
 }
 
-- 
2.54.0