Re: [Qemu-devel] [QEMU PATCH v5 1/6] migration: alternative way to set instance_id in SaveStateEntry

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

* David Gibson (david@gibson.dropbear.id.au) wrote:
> On Wed, Oct 05, 2016 at 09:44:57AM -0700, Jianjun Duan wrote:
> > Please see comments below:
> > 
> > On 10/05/2016 03:12 AM, Dr. David Alan Gilbert wrote:
> > > * Jianjun Duan (duanj@linux.vnet.ibm.com) wrote:
> > >> In QOM(QEMU Object Model) migrated objects are identified with instance_id
> > >> which is calculated automatically using their path in the QOM composition
> > >> tree. For some objects, this path could change from source to target in
> > >> migration. To migrate such objects, we need to make sure the instance_id does
> > >> not change from source to target. We add a hook in DeviceClass to do customized
> > >> instance_id calculation in such cases.
> > > 
> > > Can you explain a bit about why the path changes from source to destination;
> > > the path here should be a feature of the guest state not the host, and so I
> > > don't understand why it changes.
> > Please see the discussion with David in the previous versions:
> > http://lists.nongnu.org/archive/html/qemu-ppc/2016-06/msg00062.html
> 
> Um.. your description above really isn't an accurate summary of that
> discussion.
> 
> The point is not that the qom path will vary from source to
> destination for some arbitrary reason, but rather that we anticipate
> future changes in the QOM structure.  Specifically we're considering
> eliminating the DRC objects, and folding their (limited) state into an
> array in the parent object (either the machine or a PCI host bridge).
> 
> That would change the qom paths, and hence the auto-generated instance
> ids, which would break migration between qemu versions before and
> after the restructure.
> 
> I'm not sure that changing the instance ids is enough though, anyway,
> since we're talking about eliminating the object entirely, the
> class/type information in the migration stream also wouldn't match.
> 
> Dave, if you have ideas on how to deal with that, I'd love to hear
> them

Eliminating the object entirely would be tricky to deal with;
allowing the structure to change would work if instead of a custom instance_id
you had a custom idstr.

However, the question then becomes why is the structure changing so much;
ideally things in the migration stream should represent some tangible object
that corresponds to something real, but (again ideally) the contents
of the stream should reflect the state of those objects not the current
implementation - so if you want to change the implementation the stream
doesn't change.  Is it your implementation, or the understanding of what
the objects actually represent that's in flux?

Dave

> 
> > 
> > >> As a result, in these cases compat will not be set in the concerned
> > >> SaveStateEntry. This will prevent the inconsistent idstr to be sent over in
> > >> migration. We could have set alias_id in a similar way. But that will be
> > >> overloading the purpose of alias_id.
> > >>
> > >> The first application will be setting instance_id for DRC using its unique
> > >> index. Doing this makes the instance_id of DRC to be consistent across migration
> > >> and supports flexible management of DRC objects in migration.
> > > 
> > > Is there a reason to use a custom instance_id rather than a custom idstr
> > 
> > It can be done either way. But it is easier to deal with a integer than
> > a string.
> 
> A bit, but I don't think that's a good enough reason to introduce a
> second mechanism for overriding instance id allocations.
> 
> -- 
> David Gibson			| I'll have my music baroque, and my code
> david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
> 				| _way_ _around_!
> http://www.ozlabs.org/~dgibson


--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK