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

[Markus Armbruster <armbru@redhat.com>]

Daniel P. Berrangé <berrange@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@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.