Re: [Qemu-devel] [PATCH 09/10] s390x/css: turn on channel subsystem migration

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Halil Pasic (pasic@linux.vnet.ibm.com) wrote:
> 
> 
> On 05/08/2017 07:27 PM, Dr. David Alan Gilbert wrote:
> > * Halil Pasic (pasic@linux.vnet.ibm.com) wrote:
> >> Turn on migration for the channel subsystem and the new scheme for
> >> migrating virtio-ccw proxy devices (instead of letting the transport
> >> independent child device migrate it's proxy, use the usual
> >> DeviceClass.vmsd mechanism) for future machine versions.
> 
> [..]
> 
> >> +void css_register_vmstate(void)
> >> +{
> >> +    vmstate_register(NULL, 0, &vmstate_css, &channel_subsys);
> >> +}
> >> +
> > 
> > Why isn't that attached to a device vmsd? 
> 
> Because there is no device. The channel subsystem is not modeled 
> as a QEMU device but it does have state which needs to be
> migrated.

Ah OK.

> [..]
> 
> >> @@ -1365,6 +1373,11 @@ static void virtio_ccw_device_plugged(DeviceState *d, Error **errp)
> >>      sch->id.cu_model = virtio_bus_get_vdev_id(&dev->bus);
> >>  
> >>  
> >> +    /* Avoid generating unknown section for legacy migration target. */
> >> +    if (!css_migration_enabled()) {
> >> +        DEVICE_GET_CLASS(ccw_dev)->vmsd = NULL;
> >> +    }
> >> +
> > 
> > That's a very odd thing to do; can't you use a .needed at the
> > top level of the vmstate_virtio_ccw_dev to avoid having to
> > set it to NULL like this?
> > 
> 
> I agree it's odd. As far as I remember I can't use .needed but
> I will double check.

You can have top level .needed's - both vmstate_globalstate in
migration.c and colo's colo_state use them.

Dave

> Many thanks for your review!
> 
> Halil
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK