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

[Markus Armbruster <armbru@redhat.com>]

Markus Armbruster <armbru@redhat.com> writes:

> 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).

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.