Re: [PATCH net] virtio-net: enable big mode correctly

[Michael S. Tsirkin]

On Thu, Nov 25, 2021 at 02:05:47PM +0800, Jason Wang wrote:
> When VIRTIO_NET_F_MTU feature is not negotiated, we assume a very
> large max_mtu. In this case, using small packet mode is not correct
> since it may breaks the networking when MTU is grater than
> ETH_DATA_LEN.
> 
> To have a quick fix, simply enable the big packet mode when
> VIRTIO_NET_F_MTU is not negotiated.

This will slow down dpdk hosts which disable mergeable buffers
and send standard MTU sized packets.

> We can do optimization on top.

I don't think it works like this, increasing mtu
from guest >4k never worked, we can't regress everyone's
performance with a promise to maybe sometime bring it back.

> Reported-by: Eli Cohen <elic@xxxxxxxxxx>
> Cc: Eli Cohen <elic@xxxxxxxxxx>
> Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
>
> ---
>  drivers/net/virtio_net.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 7c43bfc1ce44..83ae3ef5eb11 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -3200,11 +3200,12 @@ static int virtnet_probe(struct virtio_device *vdev)
>  		dev->mtu = mtu;
>  		dev->max_mtu = mtu;
>  
> -		/* TODO: size buffers correctly in this case. */
> -		if (dev->mtu > ETH_DATA_LEN)
> -			vi->big_packets = true;
>  	}
>  
> +	/* TODO: size buffers correctly in this case. */
> +	if (dev->max_mtu > ETH_DATA_LEN)
> +		vi->big_packets = true;
> +
>  	if (vi->any_header_sg)
>  		dev->needed_headroom = vi->hdr_len;
>  
> -- 
> 2.25.1