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

[Jason Wang <jasowang@redhat.com>]

On Wed, May 21, 2025 at 3:45 PM Laurent Vivier <lvivier@redhat.com> wrote:
>
> On 21/05/2025 03:01, Jason Wang wrote:
> > On Tue, May 20, 2025 at 7:05 PM 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 (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@redhat.com>
> >> Signed-off-by: Laurent Vivier <lvivier@redhat.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 "+"
>
> I agree, but I kept the original syntax used everywhere in the file. It eases the search
> of the value in the file.
>
> >
> >> +               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@redhat.com>
> >
> > (Maybe we can proceed on don't stall if we had at least 1 left if
> > indirect descriptors are supported).
>
> But in this case, how to know when to stall the queue?

I meant something like:

        if (sq->vq->num_free < virito_has_feature(INDIRECT) ? 1 :
2+MAX_SKB_FRAGS) {
        }

This might be useful for the case where MAX_SKB_FRAGS is greater than
17 as well.

(But it's an independent topic anyhow)

Thanks

>
> Thank,
> Laurent
> >
> > Thanks
> >
>