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

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Date:   Sat, 6 May 2023 10:50:17 +0800
From:   Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
To:     Hao Chen <chenh@...ur.tech>
Cc:     huangml@...ur.tech, zy@...ur.tech,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        virtualization@...ts.linux-foundation.org (open list:VIRTIO CORE AND
        NET DRIVERS),
        netdev@...r.kernel.org (open list:NETWORKING DRIVERS),
        linux-kernel@...r.kernel.org (open list)
Subject: Re: [PATCH] virtio_net: set default mtu to 1500 when 'Device maximum MTU' bigger than 1500

On Sat,  6 May 2023 10:15:29 +0800, Hao Chen <chenh@...ur.tech> 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,

You mean tx packet?

If yes, I do not think this is the problem of driver.

Maybe you should give more details about the discard.

> so I changed the MTU to a more
> general 1500 when 'Device maximum MTU' bigger than 1500.
>
> Signed-off-by: Hao Chen <chenh@...ur.tech>
> ---
>  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)

s/1500/ETH_DATA_LEN/

Thanks.

> +			dev->mtu = 1500;
> +		else
> +			dev->mtu = mtu;
>  		dev->max_mtu = mtu;
>  	}
>
> --
> 2.27.0
>