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Subject: Re: [PATCH v9 21/22] KVM: s390: CPU model support for AP
 virtualization
To:     David Hildenbrand <david@redhat.com>, pmorel@linux.ibm.com,
        Tony Krowiak <akrowiak@linux.vnet.ibm.com>,
        linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
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From:   Tony Krowiak <akrowiak@linux.ibm.com>
Date:   Wed, 22 Aug 2018 17:05:31 -0400
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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
>>
>>
>>
>>
>