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

[Laurent Vivier <lvivier@redhat.com>]

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?

Thank,
Laurent
> 
> Thanks
>