RE: VIRTIO_NET_F_MTU not negotiated

[Eli Cohen]

> From: Jason Wang <jasowang@redhat.com>
> Sent: Thursday, July 28, 2022 5:09 AM
> To: Eli Cohen <elic@nvidia.com>
> Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; 
> Michael S. Tsirkin <mst@redhat.com>;
> virtualization@lists.linux-foundation.org
> Subject: Re: VIRTIO_NET_F_MTU not negotiated
> 
> On Wed, Jul 27, 2022 at 2:52 PM Eli Cohen <elic@nvidia.com> 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.
> 
> I wonder whether it's worth to bother:
> 
> 1) mgmt (above libvirt) should have the knowledge to prepare the correct XML
> 2) it's not specific to MTU, we had other features work like, for
> example, the multiqueue?
> 


Currently libvirt does not recognize setting the mtu through XML for vdpa 
device. So you mean the fix should go to libvirt?
Furthermore, even if libvirt supports MTU configuration for a vdpa device, the 
actual value provided will be ignored and the limitation will be taken from 
what the vdpa device published in its virtio_net_config structure. That makes 
the XML configuration binary.

> Thanks