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Subject: Re: [PATCH v9 21/22] KVM: s390: CPU model support for AP
 virtualization
To:     pmorel@linux.ibm.com, Tony Krowiak <akrowiak@linux.vnet.ibm.com>,
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From:   David Hildenbrand <david@redhat.com>
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Date:   Wed, 22 Aug 2018 18:57:31 +0200
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>>>
>>> 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.

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
> 
> 
> 
> 


-- 

Thanks,

David / dhildenb