Eduardo Habkost <ehabk...@redhat.com> writes: > On Tue, May 07, 2019 at 07:07:04AM +0200, Markus Armbruster wrote: >> Eduardo Habkost <ehabk...@redhat.com> writes: >> >> > This series adds machine type deprecation information to the >> > output of the `query-machines` QMP command. With this, libvirt >> > and management software will be able to show this information to >> > users and/or suggest changes to VM configuration to avoid >> > deprecated machine types. >> >> This overlaps with something I want to try, namely using Kevin's >> proposed QAPI feature flags for deprecation markings. Let's compare the >> two. >> >> To mark something as deprecated with your patches, you add a >> @support-status member somewhere, where "somewhere" is related to >> "something" by "provides information on". >> >> Example: MachineInfo (returned by query-machines) provides information >> on possible values of -machine parameter type. If -machine was >> QAPIfied, it would provide information on possible values of a QAPI >> object type's member. The type might be anonymous. The member should >> be an enum (we currently use 'str' in MachineInfo). > > QAPIfying -machine, -cpu, and -device would be wonderful. > >> >> Example: say we want to deprecate block driver "vfat", >> i.e. BlockdevDriver member @vfat. Type BlockdevDriver is used in >> multiple places; let's ignore all but BlockdevOptions. We need to add >> @support-status to something that provides information on >> BlockdevDriver, or maybe on BlockdevOptions. There is no ad hoc query >> providing information on either of the two, because QAPI/QMP >> introspection has been sufficient. What now? >> >> Can we add deprecation information to (general) QAPI/QMP introspection > > Yes, we can. I think it's a good idea. But: > >> instead of ad hoc queries? > > I'm not sure about the "instead of" part. I don't want perfect > to be the enemy of done, and I don't want QAPIfication of > -machine to be a requirement to start reporting machine type > deprecation information.
Valid point. Still, I believe we should at least try to predict how the pieces we create now would fit with the pieces we plan to create later on. Note that full QAPIfication of -machine isn't necessary to make QAPI feature "deprecated" work for machine types. Turning MachineInfo member @name into an enum, so we can tack "deprecated" onto its values, would suffice. Such a QAPIfication of machine types is still hard: QOM types are defined at compile time just like the QAPI schema, but their definition is distributed, and collected into one place only at run time. I discussed this on slide 39 of my "QEMU interface introspection: From hacks to solutions" talk (KVM Form 2015). Just for device_add, but it's just a special case of QOM. Choices listed there: * Collect drivers at compile time? Hard... * Make QAPI schema dynamic? Hard... * Forgo driver-specific arguments in schema? Defeats introspection... I'd like to add to the last item: Provide QOM introspection on par with QAPI schema introspection The QOM introspection we have (qom-list-types etc. is not on par. Back to exposing machine type deprecation. I'm doubtful your proposed solution can be applied widely. It relies on adding @support-status to something that provides information on whatever is deprecated. The initial use is with a something that is an ad hoc query, namely query-machines. To use it, the management application needs to understand what query-machines' @support-status applies to. Certainly feasible. But I fear every use will be a special case. Furthermore, a suitable ad hoc query need not exist. What then? Create suitable ad hoc queries just for communicating deprecation? Instead, I'd like us to think about a more genral solution. Or perhaps two: one for properly QAPIfied stuff, and one for QOM. >> Kevin's proposed QAPI feature flags[*] extend the QAPI language so that >> struct types can optionally have a list of feature flags, which are >> strings. Struct types suffice for his immediate needs. I'd like to use >> feature flags to mark deprecation by tacking a "deprecated" feature onto >> whatever is deprecated. This obviously needs feature support for >> everything we want to be able to deprecate: commands, and events, as >> well as members of enum and object types. >> >> Example: to deprecate block driver "vfat", add feature "deprecated" to >> BlockdevDriver member @vfat. >> >> Unlike your patches, this does not require finding a "somewhere" that >> provides information on "something". You simply tack "deprecated" right >> onto "something". >> >> Your patches provide more information, however: human-readable messages. > > It also includes a machine-friendly suggested alternative (which > I think is even more important that the human-readable message). I agree we should point to a preferred replacement whenever we deprecate something. We have to do it in documentation. And we generally do, in qemu-deprecated.texi. How useful would doing it in QMP as well be? Depends on what management applications can do with the additional information. > We could extend QAPI introspection to return that if necessary, > right? I'm confident we can come up with *something*. It might kill the neat and simple "use QAPI features to communicate deprecation" idea, though. >> Food for thought :) >> >> >> [*] Hiding in >> Subject: [PATCH 0/4] file-posix: Add dynamic-auto-read-only QAPI feature >> Date: Mon, 8 Apr 2019 16:35:39 +0200 >> Message-Id: <20190408143543.3982-1-kw...@redhat.com>
