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

[Daniel P . Berrangé]

On Tue, Feb 14, 2023 at 12:57:28PM +0100, Markus Armbruster wrote:
> 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 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.

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.

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.

With regards,
Daniel
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