Re: [PATCH] virtio_net: set default mtu to 1500 when 'Device maximum MTU' bigger than 1500

Michael S. Tsirkin
2023-05-07T06:00:38Z

On Sat, May 06, 2023 at 10:15:29AM +0800, Hao Chen wrote:
> When VIRTIO_NET_F_MTU(3) Device maximum MTU reporting is supported.
> If offered by the device, device advises driver about the value of its
> maximum MTU. If negotiated, the driver uses mtu as the maximum
> MTU value. But there the driver also uses it as default mtu,
> some devices may have a maximum MTU greater than 1500, this may
> cause some large packages to be discarded, so I changed the MTU to a more
> general 1500 when 'Device maximum MTU' bigger than 1500.
>
> Signed-off-by: Hao Chen <chenh@yusur.tech>

I don't see why not use the maximum. Bigger packets = better
performance.

> ---
> drivers/net/virtio_net.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 8d8038538fc4..e71c7d1b5f29 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -4040,7 +4040,10 @@ static int virtnet_probe(struct virtio_device *vdev)
> goto free;
> }
>
> - dev->mtu = mtu;
> + if (mtu > 1500)
> + dev->mtu = 1500;
> + else
> + dev->mtu = mtu;
> dev->max_mtu = mtu;
> }
>
> --
> 2.27.0