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

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

On 3/18/22 00:43, David Gibson wrote:
> 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 info. qdev_set_legacy_instance_id() was the missing piece I was
looking for to continue with the dc->vmsd transition I'd like to do.


> 
> 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.
> 

This is the commit that introduced DRC migration:


commit a50919dddf148b0a2008db4a0593dbe69e1059c0
Author: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>
Date:   Mon May 22 16:35:49 2017 -0300

     hw/ppc: migrating the DRC state of hotplugged devices


I'd say you can cut yourself some slack this time. Blame that guy instead.




Thanks,


Daniel