Re: [PATCH v4 14/16] qapi: deprecate "device" field of DEVICE_* events
Daniel P. Berrangé <berrange(a)redhat.com> 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> 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.