Re: VIRTIO_NET_F_MTU not negotiated

[Jason Wang]

On Thu, Jul 28, 2022 at 2:48 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Thu, Jul 28, 2022 at 01:51:37PM +0800, Jason Wang wrote:
> > On Thu, Jul 28, 2022 at 1:39 PM Eli Cohen <elic@nvidia.com> wrote:
> > >
> > > > 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?
> >
> > Probably.
> >
> > > 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.
> >
> > Yes, we suffer from a similar issue for "queues=". I think we should
> > fix qemu by failing the initialization if the value provided by cli
> > doesn't match what is read from config space.
> >
> > E.g when mtu=9000 was set by cli but the actual mtu is 1500.
> >
> > Thanks
>
>
> Jason most features are passthrough now, no?
> Why do you want to make this one special?

I don't want to make anything special, but I couldn't find a better approach.

MTU is not the only thing. It applies to all the other features whose
default value is false (MQ, RSS, etc).

Thanks

>
>
>
> > >
> > > > Thanks
> > >
>