RE: VIRTIO_NET_F_MTU not negotiated

[Eli Cohen]

> -----Original Message-----
> From: Michael S. Tsirkin <mst@redhat.com>
> Sent: Wednesday, July 27, 2022 10:25 AM
> To: Eli Cohen <elic@nvidia.com>
> Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Jason 
> Wang <jasowang@redhat.com>;
> virtualization@lists.linux-foundation.org
> Subject: Re: VIRTIO_NET_F_MTU not negotiated
> 
> 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.
> 

How about this one:

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 1067e72b3975..e464e4645c79 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -3188,6 +3188,7 @@ static void virtio_net_guest_notifier_mask(VirtIODevice 
*vdev, int idx,
 static void virtio_net_set_config_size(VirtIONet *n, uint64_t host_features)
 {
     virtio_add_feature(&host_features, VIRTIO_NET_F_MAC);
+    virtio_add_feature(&host_features, VIRTIO_NET_F_MTU);

     n->config_size = virtio_feature_get_config_size(feature_sizes,
                                                     host_features);
@@ -3512,6 +3513,7 @@ static void virtio_net_device_realize(DeviceState *dev, 
Error **errp)

    if (nc->peer && nc->peer->info->type == NET_CLIENT_DRIVER_VHOST_VDPA) {
         struct virtio_net_config netcfg = {};
+        n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
         memcpy(&netcfg.mac, &n->nic_conf.macaddr, ETH_ALEN);
         vhost_net_set_config(get_vhost_net(nc->peer),
             (uint8_t *)&netcfg, 0, ETH_ALEN, VHOST_SET_CONFIG_TYPE_MASTER);