On Tue, Oct 26, 2021 at 05:15:10PM +0200, Markus Armbruster wrote:
Daniel P. Berrangé <berrange(a)redhat.com> writes:
> On Tue, Oct 26, 2021 at 11:37:19AM +0200, Markus Armbruster wrote:
>> Kevin Wolf <kwolf(a)redhat.com> writes:
>>
>> > Am 25.10.2021 um 07:25 hat Markus Armbruster geschrieben:
>> >> By convention, names starting with "x-" are experimental.
The parts
>> >> of external interfaces so named may be withdrawn or changed
>> >> incompatibly in future releases.
>> >>
>> >> Drawback: promoting something from experimental to stable involves a
>> >> name change. Client code needs to be updated.
>> >>
>> >> Moreover, the convention is not universally observed:
>> >>
>> >> * QOM type "input-barrier" has properties
"x-origin", "y-origin".
>> >> Looks accidental, but it's ABI since 4.2.
>> >>
>> >> * QOM types "memory-backend-file",
"memory-backend-memfd",
>> >> "memory-backend-ram", and "memory-backend-epc"
have a property
>> >> "x-use-canonical-path-for-ramblock-id" that is documented
to be
>> >> stable despite its name.
>> >>
>> >> We could document these exceptions, but documentation helps only
>> >> humans. We want to recognize "unstable" in code, like
"deprecated".
>> >>
>> >> Replace the convention by a new special feature flag
"unstable". It
>> >> will be recognized by the QAPI generator, like the existing feature
>> >> flag "deprecated", and unlike regular feature flags.
>> >>
>> >> This commit updates documentation and prepares tests. The next commit
>> >> updates the QAPI schema. The remaining patches update the QAPI
>> >> generator and wire up -compat policy checking.
>> >>
>> >> Signed-off-by: Markus Armbruster <armbru(a)redhat.com>
>> >
>> > Obviously, replacing the old convention gets rid of the old drawbacks,
>> > but adds a new one: While using x- makes it very obvious for a human
>> > user that this is an unstable feature, a feature flag in the schema will
>> > almost certainly go unnoticed in manual use.
>>
>> I thought about this, but neglected to put it in writing. My bad.
>>
>> Manual use of unstable interfaces is mostly fine. Human users can adapt
>> to changing interfaces. HMP works that way.
>>
>> Management applications are better off with a feature flag than with a
>> naming convention we sometimes ignore.
>
> We will sometimes ignore/forget the feature flag too though, so I'm
> not convinced there's much difference there.
-compat unstable-input=reject,unstable-output=hide should help you stay
on the straight & narrow :)
That's from the pov of the mgmt app. I meant from the POV of QEMU
maintainers forgetting to add "unstable" flag, just as they might
forget to add a "x-" prefix.
Regards,
Daniel
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