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

Daniel P. Berrangé <berrange(a)redhat.com&gt; writes: > On Tue, Feb 14, 2023 at 12:57:28PM +0100, Markus Armbruster wrote: >> Daniel P. Berrangé <berrange(a)redhat.com&gt; writes: >> >>> On Tue, Feb 14, 2023 at 09:54:22AM +0100, Markus Armbruster wrote: >>>> Daniel P. Berrangé <berrange(a)redhat.com&gt; 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.