* 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.