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

[Markus Armbruster]

Daniel P. Berrangé <berrange@redhat.com> writes:

> On Tue, Feb 14, 2023 at 09:54:22AM +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.
>
> 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 the mgmt app is using IDs everywhere when dealing with a device,
> then trap effectively doesn't exist for their usage scenario.