Re: VIRTIO_NET_F_MTU not negotiated

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jul 27, 2022 at 06:51:56AM +0000, Eli Cohen wrote:
> I found out that the reason why I could not enforce the mtu stems from the fact that I did not configure max mtu for the net device (e.g. through libvirt <mtu size="9000"/>).
> Libvirt does not allow this configuration for vdpa devices and probably for a reason. The vdpa backend driver has the freedom to do it using its copy of virtio_net_config.
> 
> The code in qemu that is responsible to allow to consider the device MTU restriction is here:
> 
> static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> {
>     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
>     VirtIONet *n = VIRTIO_NET(dev);
>     NetClientState *nc;
>     int i;
> 
>     if (n->net_conf.mtu) {
>         n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
>     }
> 
> The above code can be interpreted as follows:
> if the command line arguments of qemu indicates that mtu should be limited, then we would read this mtu limitation from the device (that actual value is ignored).
> 
> I worked around this limitation by unconditionally setting VIRTIO_NET_F_MTU in the host features. As said, it only indicates that we should read the actual limitation for the device.
> 
> If this makes sense I can send a patch to fix this.

Well it will then either have to be for vdpa only, or have
compat machinery to avoid breaking migration.

-- 
MST