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Date:   Thu, 25 Nov 2021 09:09:39 +0200
From:   Eli Cohen <elic@nvidia.com>
To:     Jason Wang <jasowang@redhat.com>
CC:     <mst@redhat.com>, <virtualization@lists.linux-foundation.org>,
        <netdev@vger.kernel.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH net] virtio-net: enable big mode correctly
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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. We can do optimization on top.
> 
> Reported-by: Eli Cohen <elic@nvidia.com>
> Cc: Eli Cohen <elic@nvidia.com>
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  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 VIRTIO_NET_F_MTU is provided, then dev->max_mtu is going to equal
ETH_DATA_LEN (will be set in ether_setup()) so I don't think it will set
big_packets to true.


>  	if (vi->any_header_sg)
>  		dev->needed_headroom = vi->hdr_len;
>  
> -- 
> 2.25.1
>