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 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. -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson
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