Re: [PATCH v9 21/22] KVM: s390: CPU model support for AP virtualization

[Tony Krowiak]

On 08/22/2018 12:57 PM, David Hildenbrand wrote:
> > > > In this case we will have no problem with older guests not having idea
> > > > about APXA.
> > > >
> > > > Would it be a solution?
> > > Any feature the guest sees, should be part of the CPU model. The whole
> > > environment for cpu subfunctions is already in place both in KVM and
> > > QEMU. Only disabling subfunctions in KVM is not implemented yet.
> > >
> > > You can exclude any subfunctions/facilities that are only valid on LPAR
> > > level and cannot be used in some guest either way. (that makes life
> > > sometimes easier)
> > >
> > >
> > > I know that this might sound a little bit complicated, but it really
> > > isn't. Boils down to modifying kvm_s390_cpu_feat_init() and specifying
> > > some features+feature groups in QEMU.
> > OK, we definitively need another patch/patch-set, to handle this.
> > Do you think it can be done in another series since if we always support
> > APXA when we have AP instructions, we already have an indication that
> > APXA exist: the AP facility.
> Please implement the subfunction stuff right away. This will allow to
> handle all future facilities transparently from a kernel POV.

I find your use of the term 'subfunction' confusing here. In the
kvm_s390_cpu_feat_init(void) function, it looks like the
kvm_s390_available_subfunc structure is filled in with bits
returned from CPACF queries of various MSA facilities to indicate
which CPACF functions are supported. APXA is not a subfunction but
a facility that is indicated by a bit returned from the PQAP(QCI)
instruction. If we are to implement this, wouldn't it be done as
a CPU model feature as opposed to a subfunction? Am I
misunderstanding what you are asking for?

> Implementing that should be easy - and I don't like gluing features
> together in such a way.
>
> You can always assure that consistent data (e.g. AP + APXA availability)
> is reported from KVM to QEMU.
>
> > Regards,
> > Pierre