Re: [PATCH 0/4] hw/s390x: Alias @dump-skeys -> @dump-s390-skey and deprecate

[Dr. David Alan Gilbert]

* Markus Armbruster (armbru@redhat.com) wrote:
> Philippe Mathieu-Daudé <philmd@linaro.org> writes:
> 
> > Hi Daniel, Dave, Markus & Thomas.
> >
> > On 4/6/24 06:58, Markus Armbruster wrote:
> >> "Dr. David Alan Gilbert" <dave@treblig.org> writes:
> >>> * Daniel P. Berrangé (berrange@redhat.com) wrote:
> >>>> On Fri, May 31, 2024 at 06:47:45AM +0200, Thomas Huth wrote:
> >>>>> On 30/05/2024 09.45, Philippe Mathieu-Daudé wrote:
> >>>>>> We are trying to unify all qemu-system-FOO to a single binary.
> >>>>>> In order to do that we need to remove QAPI target specific code.
> >>>>>>
> >>>>>> @dump-skeys is only available on qemu-system-s390x. This series
> >>>>>> rename it as @dump-s390-skey, making it available on other
> >>>>>> binaries. We take care of backward compatibility via deprecation.
> >>>>>>
> >>>>>> Philippe Mathieu-Daudé (4):
> >>>>>>     hw/s390x: Introduce the @dump-s390-skeys QMP command
> >>>>>>     hw/s390x: Introduce the 'dump_s390_skeys' HMP command
> >>>>>>     hw/s390x: Deprecate the HMP 'dump_skeys' command
> >>>>>>     hw/s390x: Deprecate the QMP @dump-skeys command
> >>>>>
> >>>>> Why do we have to rename the command? Just for the sake of it? I think
> >>>>> renaming HMP commands is maybe ok, but breaking the API in QMP is 
> >>>>> something
> >>>>> you should consider twice.
> >
> > I'm looking at how to include this command in the new "single binary".
> >
> > Markus explained in an earlier series, just expanding this command as
> > stub to targets that don't implement it is not backward compatible and
> > breaks QMP introspection. Currently on s390x we get a result, on other
> > targets the command doesn't exist. If we add a stubs, then other targets
> > return something (even if it is an empty list), confusing management
> > interface.
> 
> Loss of introspection precision is a concern, not a hard "no".
> 
> We weigh all the concerns, and pick a solution we hate the least :)
> 
> > So this approach use to deprecate process to include a new command
> > which behaves differently on non-s390x targets.
> >
> > If we don't care for this particular case, better. However I'd still
> > like to discuss this approach for other target-specific commands.
> >
> >> PRO rename: the command's tie to S390 is them immediately obvious, which
> >> may be useful when the command becomes available in qemu-systems capable
> >> of running other targets.
> >>
> >> CON rename: users need to adapt.
> >>
> >> What are the users?  Not libvirt, as far as I can tell.
> >
> > Years ago we said, "all HMP must be based on QMP".
> 
> In practice, it's closer to "HMP must be base on QMP when the
> functionality does or should exist in QMP."
> 
> >                                                    Now we realize HMP
> > became stable because QMP-exposed, although not consumed externally...
> 
> I'm afraid I didn't get this part.
> 
> > Does the concept of "internal QMP commands" makes sense for HMP debug
> > ones? (Looking at a way to not expose them). We could use the "x-"
> > prefix to not care about stable / backward compat, but what is the point
> > of exposing to QMP commands that will never be accessed there?
> >
> >>>> That was going to be my question too. Seems like its possible to simply
> >>>> stub out the existing command for other targets.
> >>
> >> That's going to happen whether we rename the commands or not.
> >> 
> >>> Are these commands really supposed to be stable, or are they just debug
> >>> commands?  If they are debug, then add the x- and don't worry too much.
> >
> > OK.
> >
> >> docs/devel/qapi-code-gen.rst:
> >>
> >>      Names beginning with ``x-`` used to signify "experimental".  This
> >>      convention has been replaced by special feature "unstable".
> >>
> >> Feature "unstable" is what makes something unstable, and is what
> >> machines should check.
> >
> > What I mentioned earlier could be 'Feature "internal" or "debug"'.
> 
> What's the difference to "unstable"?

It should be clear *why* something is marked x- - something that's
marked 'x-' because the feature is still in development is expected to shake
out at some point, and the interface designed so it can.
(and at some point the developer should get a prod to be asked whethere the
x- can be removed).
That's different from it permenantly being x- because it's expected to
change as the needs of the people debugging change.

Dave

> >> An "x-" prefix may still be useful for humans.  Machines should *not*
> >> key on the prefix.  It's unreliable anyway: InputBarrierProperties
> >> member @x-origin is stable despite it's name.  Renames to gain or lose
> >> the prefix may or may not be worth the bother.
> >
> > Could follow the rules and be renamed as "origin-coordinate-x".
> 
> I don't think it's worth the trouble.  The "x-" prefix is now strictly
> for humans, and humans can figure out what the x- in @x-origin,
> @y-origin means.
> 
> [...]
> 
-- 
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