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

[Daniel P . Berrangé]

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.

If the mgmt app is using IDs everywhere when dealing with a device,
then trap effectively doesn't exist for their usage scenario.

With regards,
Daniel
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