Re: [Qemu-devel] [libvirt] [PATCH 7/9] qmp: Add runnability information to query-cpu-definitions

[Eduardo Habkost]

On Tue, May 31, 2016 at 03:24:50PM +0200, Markus Armbruster wrote:
> Eduardo Habkost <address@hidden> writes:
> 
> > On Mon, May 30, 2016 at 11:33:38AM +0200, Markus Armbruster wrote:
> [...]
> >> The new members encode an answer to the question whether a certain CPU
> >> usable with the current machine an accelerator, and if no, why.
> >> The possible answers are:
> >> 
> >> (1) Don't know.
> >> (2) Yes.
> >> (3) No, but we can't say why.
> >> (4) No, and here's a list of reasons.
> >> 
> >> The two "dunno" answers (1) and (3) exist so we don't have to boil the
> >> CPU ocean now.
> >> 
> >> Without them, the natural solution is a single member, where (4) is
> >> encoded as nonempty list, and (2) could be encoded as empty list or
> >> absent.
> >> 
> >> Now let me try to fit in (1) and (3).
> >> 
> >> The obvious way to do (1) is absent.  So let's use empty list for (2).
> >> 
> >> That leaves (3).  I think the simplest solution that could possibly work
> >> is to treat it as a special "dunno" reason: encode it just like (4), but
> >> with a special "dunno" list element.  I'd use the empty string.
> >> 
> >> Could even be used if we need to distinguish
> >> 
> >> (4a) No, and here's the *complete* list of reasons.
> >> (4b) No, and here's a possibly incomplete list of reasons.
> >> 
> >> For (4b), include the "dunno" element with the others.
> >> 
> >> Unlike the proposed solution, this one doesn't leave interface crud
> >> behind if we succeed in getting rid of (1) and (3):
> >> 
> >> * When (1) goes away, the single member becomes mandatory.
> >> 
> >> * When (3) goes away, the special "dunno" list element no longer occurs.
> >
> > I like your suggestion.
> >
> > I suggest "type" as the "dunno" element. It would keep the
> > existing "QOM property name" semantics, and it would just mean
> > "sorry, the only advice we can currently give you is to choose a
> > different CPU type". It even matches the previous documentation I
> > sent describing the meaning of read-only property names.
> >
> > Rewriting the docs again:
> >
> >  # Virtual CPU definition.
> >  #
> >  # @name: the name of the CPU definition
> > -# @runnable: #optional Whether the CPU model us usable with the
> > -#            current machine and accelerator. Omitted if we don't
> > -#            know the answer. (since 2.7)
> > -# @unavailable-features: #optional List of attributes that prevent
> > +# @unavailable-features: #optional List of properties that prevent
> >  #                        the CPU model from running in the current
> > -#                        host. Present only if @runnable is false.
> > -#                        (since 2.7)
> > +#                        host. (since 2.7)
> >  #
> >  # @unavailable-features is a list of QOM property names that
> >  # represent CPU model attributes that prevent the CPU from running.
> > -# If the QOM property is read-only, that means the CPU model can
> > -# never run in the current host. If the property is read-write, it
> > +# If the QOM property is read-only, that means there's no known
> > +# way to make the CPU model run in the current host.
> > +# If the property is read-write, it
> >  # means that it MAY be possible to run the CPU model in the current
> >  # host if that property is changed. Management software can use it
> >  # as hints to suggest or choose an alternative for the user, or
> >  # just to generate meaningful error messages explaining why the CPU
> >  # model can't be used.
> 
> Should we spell out the special case "type"?

It is not exactly a special case (it is a read-only property like
any other), but it's worth mentioning. I will change it to:

# If the QOM property is read-only, that means there's no known
# way to make the CPU model run in the current host. If
# absolutely no extra information will be returned to explain why
# the CPU model is not runnable, implementations may simply
# return "type" as the property name.

> 
> > +# If @unavailable-features is an empty list, the CPU model is
> > +# runnable using the current host and machine-type.
> > +# If @unavailable-features is not present, runnability
> > +# information for the CPU model is not available.
> >  #
> >  # Since: 1.2.0
> >  ##
> 
> I'm happy with this interface.  Thanks!

Thanks!

-- 
Eduardo