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

[Peter Maydell]

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().

> 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?

Only if you're prepared to break migration compatibility, I think.

-- PMM