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

[Markus Armbruster <armbru@redhat.com>]

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.

I suggested to deprecate it in review of "[PATCH v3 14/15] qapi:
introduce DEVICE_ON event" (Message-ID: <873579x67l.fsf@pond.sub.org>).
Quote:

    We commonly send both device ID and QOM path, mostly for historical
    reasons: the former precede the latter.

    There are exceptions, such as query-cpus-fast.  Can't say offhand
    whether CPUs can be created with IDs.

    [...]

    I'd be in favour of deprecating and deleting redundant device IDs in QMP
    output.