Re: Question about vmstate_register(), dc->vmsd and instance_id

[David Gibson]

On Thu, Mar 17, 2022 at 04:29:14PM +0000, Dr. David Alan Gilbert wrote:
> * Peter Maydell (peter.maydell@linaro.org) wrote:
> > On Thu, 17 Mar 2022 at 14:03, Daniel Henrique Barboza
> > <danielhb413@gmail.com> wrote:
> > > I've been looking into converting some vmstate_register() calls to use 
> > > dc->vmsd,
> > > using as a base the docs in docs/devel/migration.rst. This doc mentions 
> > > that we
> > > can either register the vmsd by using vmstate_register() or we can use 
> > > dc->vmsd
> > > for qdev-based devices.
> > >
> > > When trying to convert this vmstate() call for the qdev alternative 
> > > (hw/ppc/spapr_drc.c,
> > > drc_realize()) I found this:
> > >
> > >      vmstate_register(VMSTATE_IF(drc), spapr_drc_index(drc), 
> > > &vmstate_spapr_drc,
> > >                       drc);
> > >
> > > spapr_drc_index() is an unique identifier for these DRC devices and it's 
> > > being used
> > > as instance_id. It is not clear to me how we can keep using this same 
> > > instance_id when
> > > using the dc->vmsd alternative. By looking a bit into migration files I 
> > > understood
> > > that if dc->vmsd is being used the instance_id is always autogenerated. 
> > > Is that correct?
> > 
> > Not entirely. It is the intended common setup, but because changing
> > the ID value breaks migration compatibility there is a mechanism
> > for saying "my device is special and needs to set the instance ID
> > to something else" -- qdev_set_legacy_instance_id().
> 
> Yes, this is normally only an issue for 'system' or memory mapped
> devices;  for things hung off a bus that has it's own device naming,
> then each instance of a device has it's own device due to the bus name
> so instance_id's aren't used.  Where you've got a few of the
> same device with the same name, and no bus for them to be named by, then
> the instance_id is used to uniquify them.

Thanks for the information.  I remember deciding at the time that just
using vmsd wouldn't work for the DRCs because we needed this fixed
index.  At the time either qdev_set_legacy_instance_id() didn't exist,
or I didn't know about it, hence the explicit vmstate_register() call
so that an explicit instance id could be supplied.

-- 
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

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