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

From: Eli Cohen
Date: Thu Nov 25 2021 - 02:11:55 EST


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@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 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
>