Re: [PATCH v2 3/3] virtio_net: Enforce minimum TX ring size for reliability

[Xuan Zhuo <xuanzhuo@linux.alibaba.com>]

On Wed, 21 May 2025 11:22:36 +0200, Laurent Vivier <lvivier@redhat.com> wrote:
> 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)
>
> This patch enforces that the TX ring size ('ring_num') must be strictly
> greater than (MAX_SKB_FRAGS + 2). This ensures that the ring is
> always large enough to hold at least one maximally-fragmented packet
> plus at least one additional slot.
>
> Reported-by: Lei Yang <leiyang@redhat.com>
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>

Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>

> ---
>  drivers/net/virtio_net.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index ff4160243538..50b851834ae2 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -3481,6 +3481,12 @@ 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);
> --
> 2.49.0
>