Subject: Re: [PATCH net-next v2] virtio-net: xsk: Support wakeup on RX side

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 2 Mar 2026 13:53:08 -0500

On Mon, Mar 02, 2026 at 11:41:58PM +0700, Bui Quang Minh wrote:
> When XDP_USE_NEED_WAKEUP is used and the fill ring is empty so no buffer
> is allocated on RX side, allow RX NAPI to be descheduled. This avoids
> wasting CPU cycles on polling. Users will be notified and they need to
> make a wakeup call after refilling the ring.
> 
> Signed-off-by: Bui Quang Minh <[email-masked]>
> ---
> Changes in v2:
> - Fix the flag check in virtnet_xsk_wakeup
> - Link to v1: https://lore.kernel.org/netdev/[email-masked]/
> ---
>  drivers/net/virtio_net.c | 35 +++++++++++++++++++++++++++--------
>  1 file changed, 27 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index db88dcaefb20..3614002fc87c 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1454,8 +1454,19 @@ static int virtnet_add_recvbuf_xsk(struct virtnet_info *vi, struct receive_queue
>  	xsk_buffs = rq->xsk_buffs;
>  
>  	num = xsk_buff_alloc_batch(pool, xsk_buffs, rq->vq->num_free);
> -	if (!num)
> +	if (!num) {
> +		if (xsk_uses_need_wakeup(pool)) {
> +			xsk_set_rx_need_wakeup(pool);
> +			/* Return 0 instead of -ENOMEM so that NAPI is
> +			 * descheduled.
> +			 */
> +			return 0;

With this change, try_fill_recv() can return true even though zero RX
buffers were posted in need_wakeup mode, which contradicts its
documentation ("Returns false if we couldn't fill entirely (OOM)").

Methinks we need to at least update the comment near try_fill_recv.

> +		}
> +
>  		return -ENOMEM;
> +	} else {
> +		xsk_clear_rx_need_wakeup(pool);
> +	}
>  
>  	len = xsk_pool_get_rx_frame_size(pool) + vi->hdr_len;
>  
> @@ -1588,20 +1599,19 @@ static bool virtnet_xsk_xmit(struct send_queue *sq, struct xsk_buff_pool *pool,
>  	return sent;
>  }
>  
> -static void xsk_wakeup(struct send_queue *sq)
> +static void xsk_wakeup(struct napi_struct *napi, struct virtqueue *vq)
>  {
> -	if (napi_if_scheduled_mark_missed(&sq->napi))
> +	if (napi_if_scheduled_mark_missed(napi))
>  		return;
>  
>  	local_bh_disable();
> -	virtqueue_napi_schedule(&sq->napi, sq->vq);
> +	virtqueue_napi_schedule(napi, vq);
>  	local_bh_enable();
>  }
>  
>  static int virtnet_xsk_wakeup(struct net_device *dev, u32 qid, u32 flag)
>  {
>  	struct virtnet_info *vi = netdev_priv(dev);
> -	struct send_queue *sq;
>  
>  	if (!netif_running(dev))
>  		return -ENETDOWN;
> @@ -1609,9 +1619,18 @@ static int virtnet_xsk_wakeup(struct net_device *dev, u32 qid, u32 flag)
>  	if (qid >= vi->curr_queue_pairs)
>  		return -EINVAL;
>  
> -	sq = &vi->sq[qid];
> +	if (flag & XDP_WAKEUP_TX) {
> +		struct send_queue *sq = &vi->sq[qid];
> +
> +		xsk_wakeup(&sq->napi, sq->vq);
> +	}
> +
> +	if (flag & XDP_WAKEUP_RX) {
> +		struct receive_queue *rq = &vi->rq[qid];
> +
> +		xsk_wakeup(&rq->napi, rq->vq);
> +	}
>  
> -	xsk_wakeup(sq);
>  	return 0;
>  }
>  
> @@ -1623,7 +1642,7 @@ static void virtnet_xsk_completed(struct send_queue *sq, int num)
>  	 * wakeup the tx napi to consume the xsk tx queue, because the tx
>  	 * interrupt may not be triggered.
>  	 */
> -	xsk_wakeup(sq);
> +	xsk_wakeup(&sq->napi, sq->vq);
>  }
>  
>  static int __virtnet_xdp_xmit_one(struct virtnet_info *vi,
> -- 
> 2.43.0