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

[Philippe Mathieu-Daudé]

On 14/2/23 12:49, Markus Armbruster wrote:
> Daniel P. Berrangé <berrange at 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 at 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.

Are you suggesting this is a stable interface and we can not move
devices (like from /machine/unattached/ to /machine/peripheral/)
without going thru the deprecation process?