Re: [PATCH v2 18/19] vdpa: Add device migration blocker

[Eugenio Perez Martin]

On Fri, Jul 15, 2022 at 10:51 AM Jason Wang <jasowang@redhat.com> wrote:
>
> On Fri, Jul 15, 2022 at 1:40 PM Eugenio Perez Martin
> <eperezma@redhat.com> wrote:
> >
> > On Fri, Jul 15, 2022 at 6:03 AM Jason Wang <jasowang@redhat.com> wrote:
> > >
> > > On Fri, Jul 15, 2022 at 12:32 AM Eugenio Pérez <eperezma@redhat.com> 
> > > wrote:
> > > >
> > > > Since the vhost-vdpa device is exposing _F_LOG,
> > >
> > > I may miss something but I think it doesn't?
> > >
> >
> > It's at vhost_vdpa_get_features. As long as SVQ is enabled, it's
> > exposing VHOST_F_LOG_ALL.
>
> Ok, so this needs to be specified in the change log.

Got it, I'll write some note.

> But I'm kind of
> confused here, we do want to allow migration to work so why we disable
> it?
>

With x-svq parameter, migration of simple devices with no cvq "is
possible". It has intrinsic problems like can't emit the gratuitous
arp but it's possible and traffic can continue.

But devices with cvq require to restore the state at the destination.
That part is not implemented, so it's blocked at the moment.

In the immediate future not all cases (as "net features") will be
available: net/vhost-net.c (or virtio-net.c?) needs to know how to
inject the state at the destination to restore the guest visible
configuration. It's simple code, but it needs to be developed. So
migration blocker is kept for these features. Hopefully, we will reach
a point where all features supported by virtio-net.c will be
supported, but the right thing to do is to merge basic ones first.

Thanks!