RE: VIRTIO_NET_F_MTU not negotiated

[Eli Cohen]

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.