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

[David Hildenbrand]

> > > > 
> > > > Just FYI, on other architectures (e.g. s390x), other conditions (e.g. cpu
> > > > generation) also define if a CPU model is runnable, so the pure availability of
> > > > features does not mean that a cpu model is runnable.
> > > > 
> > > > We could have runnable=false and unavailable-features being an empty list.    
> > > 
> > > Even on those cases, can't the root cause be mapped to a QOM
> > > property name (e.g. "cpu-generation"), even if it's property that
> > > can't be changed by the user?  
> > 
> > In the current state, no.  
> 
> But it could be implemented by s390x in the future, if it's
> considered useful, right?

Yes, we could fit that into read-only properties if we would ever need it
(like cpu-generation you mentioned and cpu-ga-level - both will always be
read-only).

However we could come up with more optional fields for that in the future.
(like unsupported-values or sth. like that). I actually prefer
unavailable-features over runnability-blockers.

> 
> > 
> > So I think for runnable=false:
> > a) unavailable-features set -> can be made runnable
> > b) unavailable-features not set -> cannot be made runnable
> > 
> > would be enough.  
> 
> I understand it would be enough, but I would like to at least
> define semantics that would make sense for all architectures in
> case it gets implemented in the future. Would the proposal below
> make sense?
> 

Yes, I think so. However to really make good hints, upper layers would most
likely need more information about the exact problem with a property -
maybe something like an enum value per problematic property.
(UNAVAILABLE_FEATURE, VALUE_TOO_BIG, VALUE_TOO_SMALL, UNSUPPORTED_VALUE) ...

> > > 
> > > We could replace this with something more generic, like:
> > > 
> > > @runnability-blockers: List of attributes that prevent the CPU
> > >   model from running in the current host.
> > >   
> > >   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 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.
> > > 
> > > (I am looking for a better name than "runnability-blockers").
> > >   

Not sure which approach would be better.

David