Re: [PATCH v2 5/6] vhost: claim that we support GUEST_ANNOUNCE feature

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Tue, Dec 22, 2015 at 04:11:08PM +0800, Peter Xu wrote:
> On Thu, Dec 17, 2015 at 11:12:00AM +0800, Yuanhan Liu wrote:
> > It's actually a feature already enabled in Linux kernel. What we need to
> > do is simply to claim that we support such feature, and nothing else.
> > 
> > With that, the guest will send GARP messages after live migration.
> > 
> > Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
> > ---
> >  lib/librte_vhost/virtio-net.c | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/lib/librte_vhost/virtio-net.c b/lib/librte_vhost/virtio-net.c
> > index 03044f6..0ba5045 100644
> > --- a/lib/librte_vhost/virtio-net.c
> > +++ b/lib/librte_vhost/virtio-net.c
> > @@ -74,6 +74,7 @@ static struct virtio_net_config_ll *ll_root;
> >  #define VHOST_SUPPORTED_FEATURES ((1ULL << VIRTIO_NET_F_MRG_RXBUF) | \
> >  				(1ULL << VIRTIO_NET_F_CTRL_VQ) | \
> >  				(1ULL << VIRTIO_NET_F_CTRL_RX) | \
> > +				(1ULL << VIRTIO_NET_F_GUEST_ANNOUNCE) | \
> 
> Do we really need this? I can understand when guest declare with
> this VIRTIO_NET_F_GUEST_ANNOUNCE flag. With that, guest itself will
> handle the announcement after migration. However, how could I
> understand if it's declared by a vhost-user backend? What does it
> mean?
> 
> In vhost-user.txt (in QEMU repo docs/specs/), the only place that
> mentioned this is SEND_RARP:
> 
>  * VHOST_USER_SEND_RARP
> 
>       Id: 19
>       Equivalent ioctl: N/A
>       Master payload: u64
> 
>       Ask vhost user backend to broadcast a fake RARP to notify the migration
>       is terminated for guest that does not support GUEST_ANNOUNCE.
> 	  ...
> 
> Here, it mention the GUEST_ANNOUNCE since when guest support this,
> we do not need to send SEND_RARP to vhost-user backend again. It
> never explain what does it mean when vhost-user declaring to have
> this flag...

The actually work is done in QEMU and guest kernel. QEMU sends a config
interrupt to guest kernel when such flag is enabled after live migration
(see QEMU code virtio_net_load()). Guest kernel generates the GARP
once it receives the interrupt (see the kernel code virtnet_config_changed_work()).

Therefore, we could simply claim that we support VIRTIO_NET_F_GUEST_ANNOUNCE
feature and do nothing here.

	--yliu