linux-kernel - Re: [PATCH 2/2] virtio_net: Enforce minimum TX ring size for reliability

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Message-ID: <CACGkMEudOrbPjwLbQKXeLc9K4oSq8vDH5YD-hbrsJn1aYK6xxQ@mail.gmail.com>
Date: Wed, 21 May 2025 09:01:58 +0800
From: Jason Wang <jasowang@...hat.com>
To: Laurent Vivier <lvivier@...hat.com>
Cc: linux-kernel@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>, netdev@...r.kernel.org, 
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Subject: Re: [PATCH 2/2] virtio_net: Enforce minimum TX ring size for reliability

On Tue, May 20, 2025 at 7:05 PM Laurent Vivier <lvivier@...hat.com> wrote:
>
> The `tx_may_stop()` logic stops TX queues if free descriptors
> (`sq->vq->num_free`) fall below the threshold of (2 + `MAX_SKB_FRAGS`).
> 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 (2 + MAX_SKB_FRAGS). 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@...hat.com>
> Signed-off-by: Laurent Vivier <lvivier@...hat.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 e53ba600605a..866961f368a2 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 <= 2+MAX_SKB_FRAGS) {

Nit: space is probably needed around "+"

> +               netdev_err(vi->dev, "tx size (%d) cannot be smaller than %d\n",
> +                          ring_num, 2+MAX_SKB_FRAGS);

And here.

> +               return -EINVAL;
> +       }
> +
>         qindex = sq - vi->sq;
>
>         virtnet_tx_pause(vi, sq);
> --
> 2.49.0
>

Other than this.

Acked-by: Jason Wang <jasowang@...hat.com>

(Maybe we can proceed on don't stall if we had at least 1 left if
indirect descriptors are supported).

Thanks