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

Daniel P. Berrangé <berrange(a)redhat.com&gt; writes: > On Tue, Feb 14, 2023 at 10:25:22AM +0100, Peter Krempa wrote: >> On Tue, Feb 14, 2023 at 09:54:22 +0100, Markus Armbruster wrote: >> > Daniel P. Berrangé <berrange(a)redhat.com&gt; writes: >> > >> > > On Mon, Feb 13, 2023 at 05:01:01PM +0300, Vladimir Sementsov-Ogievskiy wrote: >> > >> The device field is redundant, because QOM path always include device >> > >> ID when this ID exist. >> > > >> > > The flipside to that view is that applications configuring QEMU are >> > > specifying the device ID for -device (CLI) / device_add (QMP) and >> > > not the QOM path. IOW, the device ID is the more interesting field >> > > than QOM path, so feels like the wrong one to be dropping. >> > >> > QOM path is a reliable way to identify a device. Device ID isn't: >> > devices need not have one. Therefore, dropping the QOM path would be >> > wrong. >> > >> > > Is there any real benefit to dropping this ? >> > >> > The device ID is a trap for the unwary: relying on it is fine until you >> > run into a scenario where you have to deal with devices lacking IDs. >> >> Note that libvirt's code is still using the 'device' bit rather than QOM >> path and the fix might not be entirely trivial although should not be >> too hard. > > 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.