On Wed, Jul 27, 2022 at 09:04:47AM +0000, Eli Cohen wrote:
> > -----Original Message-----
> > From: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > Sent: Wednesday, July 27, 2022 10:25 AM
> > To: Eli Cohen <elic@xxxxxxxxxx>
> > Cc: Eugenio Perez Martin <eperezma@xxxxxxxxxx>; qemu-devel@xxxxxxxxxx; Jason Wang <jasowang@xxxxxxxxxx>;
> > virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
> > 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);
Seems to increase config size unconditionally?
> @@ -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);
And the point is vdpa does not support migration anyway ATM, right?
--
MST
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