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

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Date: Sun, 7 May 2023 01:59:35 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Hao Chen <chenh@...ur.tech>
Cc: Jason Wang <jasowang@...hat.com>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	"open list:VIRTIO CORE AND NET DRIVERS" <virtualization@...ts.linux-foundation.org>,
	"open list:NETWORKING DRIVERS" <netdev@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>, huangml@...ur.tech,
	zy@...ur.tech
Subject: Re: [PATCH] virtio_net: set default mtu to 1500 when 'Device maximum
 MTU' bigger than 1500

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@...ur.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