On 2023/3/8 10:49, Xuan Zhuo wrote:
> If the queue of xdp xmit is not an independent queue, then when the xdp
> xmit used all the desc, the xmit from the __dev_queue_xmit() may encounter
> the following error.
>
> net ens4: Unexpected TXQ (0) queue failure: -28
>
> This patch adds a check whether sq is full in xdp xmit.
>
> Fixes: 56434a01b12e ("virtio_net: add XDP_TX support")
> Reported-by: Yichun Zhang <yichun@xxxxxxxxxxxxx>
> Signed-off-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
> Reviewed-by: Alexander Duyck <alexanderduyck@xxxxxx>
> Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> ---
> drivers/net/virtio_net.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 46bbddaadb0d..1a309cfb4976 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -767,6 +767,9 @@ static int virtnet_xdp_xmit(struct net_device *dev,
> }
> ret = nxmit;
>
> + if (!is_xdp_raw_buffer_queue(vi, sq - vi->sq))
> + check_sq_full_and_disable(vi, dev, sq);
> +
Sorry if I missed something obvious here.
As the comment in start_xmit(), the current skb is added to the sq->vq, so
NETDEV_TX_BUSY can not be returned.
/* If running out of space, stop queue to avoid getting packets that we
* are then unable to transmit.
* An alternative would be to force queuing layer to requeue the skb by
* returning NETDEV_TX_BUSY. However, NETDEV_TX_BUSY should not be
* returned in a normal path of operation: it means that driver is not
* maintaining the TX queue stop/start state properly, and causes
* the stack to do a non-trivial amount of useless work.
* Since most packets only take 1 or 2 ring slots, stopping the queue
* early means 16 slots are typically wasted.
*/
It there any reason not to check the sq->vq->num_free at the begin of start_xmit(),
if the space is not enough for the current skb, TX queue is stopped and NETDEV_TX_BUSY
is return to the stack to requeue the current skb.
It seems it is the pattern that most network driver follow, and it seems we can avoid
calling check_sq_full_and_disable() in this patch and not wasting 16 slots as mentioned
in the comment above.
> if (flags & XDP_XMIT_FLUSH) {
> if (virtqueue_kick_prepare(sq->vq) && virtqueue_notify(sq->vq))
> kicks = 1;
>
