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

[Daniel Henrique Barboza <danielhb413@gmail.com>]

Hi,

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?

Another related question is the role of instance_id per se. I understand that this
value is used to identify SaveStateEntries in migration/savevm.c and it's autogenerated
if the caller does not provide it. And there's also this comment from
register_savevm_live():

/* TODO: Individual devices generally have very little idea about the rest
    of the system, so instance_id should be removed/replaced.
    Meanwhile pass -1 as instance_id if you do not already have a clearly
    distinguishing id for all instances of your device class. */

Given that this is a 13 year old comment from Anthony Liguori I wanted to confirm its
validity. Is there a long term goal of getting rid of instance_id? Can I ignore its
role when converting these calls to dc->vmsd?


Thanks,


Daniel