Markus Armbruster <armbru@xxxxxxxxxx> writes: > Daniel P. Berrangé <berrange@xxxxxxxxxx> 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@xxxxxxxxxx> 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). Correction: that should affect only /machine/unattached/. Messing with -device and such affects /machine/peripheral-anon/. > * /machine/unattached/ > > Surrogate parent of onboard devices created without a parent. Forgot to mention: Children are named "device[N]", where N counts up from zero. > 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.
