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.mayd...@linaro.org) wrote:
On Thu, 17 Mar 2022 at 14:03, Daniel Henrique Barboza
<danielhb...@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 <danie...@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
