Re: [PATCH v4 14/16] qapi: deprecate "device" field of DEVICE_* events

On 14/2/23 13:17, Markus Armbruster wrote: > Philippe Mathieu-Daudé <philmd(a)linaro.org&gt; writes: > >> On 14/2/23 12:49, Markus Armbruster wrote: >>> Daniel P. Berrangé <berrange(a)redhat.com&gt; writes: > > [...] > >>>> What's the documented way to construct a QOM path, given only an ID as >>>> input ? >>> >>> QOM paths a gap in our documentation, even though the composition tree >>> structure has been stable since day one, and is de facto ABI. >>> >>> Short answer: "/machine/peripheral/ID". >>> >>> Long answer follows. >>> >>> We have three "containers" under /machine that serve as parents for >>> devices: >>> >>> * /machine/peripheral/ >>> >>> Parent of user-created devices with ID. Children are named "ID". >>> >>> Put there by qdev_set_id(), called from qdev_device_add_from_qdict(). >>> >>> On "user-created": Nothing stops board code to abuse qdev_set_id() for >>> onboard devices, directly or indirectly, but it really, really >>> shouldn't. >>> >>> * /machine/peripheral-anon/ >>> >>> Parent of user-created devices without ID. Children are named >>> "device[N]", where N counts up from zero. >>> >>> Put there by qdev_set_id(), called from qdev_device_add_from_qdict(). >>> >>> Again, abuse by board code is possible, but would be wrong. >>> >>> Beware: a particular device's N changes when the set of devices >>> created before it grows or shrinks. Messing with the machine type can >>> change it (different onboard devices). >>> >>> * /machine/unattached/ >>> >>> Surrogate parent of onboard devices created without a parent. >>> >>> Put there by device_set_realized() (general case), >>> qdev_connect_gpio_out_named() (input pins) , memory_region_do_init() >>> (memory regions), qemu_create_machine() (the main sysbus). >>> >>> I believe this container was created as a convenience, so we don't >>> have to retrofit parents to existing code. Probably abused ever >>> since. >> >> Are you suggesting this is a stable interface and we can not move >> devices (like from /machine/unattached/ to /machine/peripheral/) >> without going thru the deprecation process? > > Difficult question! > > The point of not changing interfaces incompatibly without a grace period > / deprecation process is not breaking users of the interface. > > When an interface has always worked a certain way, its users may well > depend on it, whether it's documented or not. > > The question to ask is always "will this break users?" > > For documented aspects, we generally assume it will. Doesn't mean we > can simply assume "won't" for undocumented aspects. > > Does this make sense? Yes, but I never considered the QOM paths as a stable interface... I'm very surprised.

"Automatically assigned to /machine/unattached/" doesn't seem quite stable...