Re: [Qemu-devel] [PATCH v2] net: Add support of VIRTIO_NET_F_GUEST_ANNOUNCE for vhost-net/vhost-user

[Jason Wang]

On 06/08/2015 04:21 PM, Thibaut Collet wrote:
> Hi,
>
> My understanding of gratuitous packet with virtio for any backend
> (vhost user or other):
> - When the VM is loaded (first start or migration) the virtio net
> interfaces are loaded ( virtio_net_load_device function in
> hw/net/virtio-net.c)
> - If the guest has the VIRTIO_NET_F_GUEST_ANNOUNCE capability, request
> to send gratuitous packet is done.
>
> 1. To enable gratuitous packet through this mechanism I have
> added VIRTIO_NET_F_GUEST_ANNOUNCE capability to hw/net/vhost_net.c. So
> host and guest can negotiate this feature when vhost-user is used.
>
> 2. self announce occurs in case of live migration. During a live
> migration a GARP is sent to all net backend through a queue dedicated
> to the net backend.
>    But for vhost-user:
>        - this operation is not possible (vhost-user has no queue)
>        - it is already done with the previous mechanism.
>    Rather to define a queue to vhost user and notify twice the guest
> to send gratuitous packet I have disable GARP from self announce and
> use only the first mechanism for that.
>
> I have tested my modifications with guest that supports
> VIRTIO_NET_F_GUEST_ANNOUNCE and vhost-user on the host. After a live
> migration I have the GARP from the guest.

Yes, your patch works well for recent drivers. But the problem is legacy
guest/driver without VIRTIO_NET_F_GUEST_ANNOUNCE. In this case there
will be no GARP sent after migration.

Thanks