[PATCH net-next] virtio-net: support xsk wake up

[Menglong Dong]

For now, XDP_RING_NEED_WAKEUP is not supported properly by the virtio-net.
Take 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.

For the tx path, we will 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.

For the rx path, we will call xsk_set_rx_need_wakeup() if we have free
buffers in rq->vq.

Race condition is considered for both rx and tx path.

Signed-off-by: Menglong Dong <dongml2@xxxxxxxxxxxxxxx>
---
 drivers/net/virtio_net.c | 52 ++++++++++++++++++++++++++++++++++------
 1 file changed, 45 insertions(+), 7 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 9b3da9f9786c..25e895b849a6 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -1323,16 +1323,27 @@ static int virtnet_add_recvbuf_xsk(struct virtnet_info *vi, struct receive_queue
 				   struct xsk_buff_pool *pool, gfp_t gfp)
 {
 	struct xdp_buff **xsk_buffs;
+	bool need_wakeup;
 	dma_addr_t addr;
 	int err = 0;
 	u32 len, i;
 	int num;
 
+	need_wakeup = xsk_uses_need_wakeup(pool);
 	xsk_buffs = rq->xsk_buffs;
 
+	/* If both rq->vq and fill ring are empty, and then the user submit
+	 * all the chunks to the fill ring and check the wake up flag
+	 * after xsk_buff_alloc_batch() and before xsk_set_rx_need_wakeup(),
+	 * we will lose the chance to wake up the rx napi, so we have to
+	 * set the need_wakeup flag here.
+	 */
+	if (need_wakeup && virtqueue_get_vring_size(rq->vq) == rq->vq->num_free)
+		xsk_set_rx_need_wakeup(pool);
+
 	num = xsk_buff_alloc_batch(pool, xsk_buffs, rq->vq->num_free);
 	if (!num) {
-		if (xsk_uses_need_wakeup(pool)) {
+		if (need_wakeup) {
 			xsk_set_rx_need_wakeup(pool);
 			/* Return 0 instead of -ENOMEM so that NAPI is
 			 * descheduled.
@@ -1341,8 +1352,6 @@ static int virtnet_add_recvbuf_xsk(struct virtnet_info *vi, struct receive_queue
 		}
 
 		return -ENOMEM;
-	} else {
-		xsk_clear_rx_need_wakeup(pool);
 	}
 
 	len = xsk_pool_get_rx_frame_size(pool) + vi->hdr_len;
@@ -1363,6 +1372,16 @@ static int virtnet_add_recvbuf_xsk(struct virtnet_info *vi, struct receive_queue
 			goto err;
 	}
 
+	if (need_wakeup) {
+		if (rq->vq->num_free)
+			/* We have free buffers, so we'd better wake up the
+			 * rx napi as soon as possible.
+			 */
+			xsk_set_rx_need_wakeup(pool);
+		else
+			xsk_clear_rx_need_wakeup(pool);
+	}
+
 	return num;
 
 err:
@@ -1440,8 +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().
@@ -1451,8 +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);
 
@@ -1470,9 +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