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

[Vladimir Sementsov-Ogievskiy]

On 14.02.23 19:28, Markus Armbruster wrote:
> Daniel P. Berrangé <berrange at redhat.com> writes:
> 
>> On Tue, Feb 14, 2023 at 12:57:28PM +0100, Markus Armbruster wrote:
>>> Daniel P. Berrangé <berrange at redhat.com> writes:
>>>
>>>> On Tue, Feb 14, 2023 at 09:54:22AM +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.
>>>>
>>>> When a mgmt app is configuring QEMU though, it does it exclusively
>>>> with device ID values. If I add a device "-device foo,id=dev0",
>>>> and then later hot-unplug it "device_del dev0", it is pretty
>>>> reasonable to then expect that the DEVICE_DELETED even will then
>>>> include the ID value the app has been using elsewhere.
>>>
>>> The management application would be well advised to use QOM paths with
>>> device_del, because only that works even for devices created by default
>>> (which have no ID), and devices the user created behind the management
>>> application's back.
>>
>> If an application is using -nodefaults, then the only devices which
>> exist will be those which are hardwired into the machine, and they
>> can't be used with device_del anyway as they're hardwired.
> 
> Your trust in the sanity of our board code is touching ;)
> 
>> So the only reason is to cope with devices created secretly by
>> the users, and that's a hairy enough problem that most apps won't
>> even try to cope with it.
> 
> Fair enough.
> 
>> At least in terms of the device hotplug area, it feels like we're
>> adding an extra hurdle for apps to solve a problem that they don't
>> actually face in practice.
>>
>> QOM paths are needed in some other QMP commands though, where
>> there is definite need to refer to devices that are hardwired,
>> most obviously qom-set/qom-get.
> 
> Also query-cpus-fast, query-hotpluggable-cpus, and possibly more I
> missed.
> 


So, finally, we don't have consensus on deprecating ids?

For me the most strong argument is that if user specify id in device_add, user should get exactly that id in DEVICE_DELETED and other events.

So if deprecate something, we'd better deprecate ids altogether, making users specify full QOM path even in device_add. But that seems quite painful for existing users with no visible benefit.

So, if no objections, I plan to resend with old "optional id & qom_path" designation for devices. We still can do a deprecation in future.

-- 
Best regards,
Vladimir