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

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Message-Id: <933db512-fca1-a471-5e2d-b247cfc94fea@linux.ibm.com>
Date:   Thu, 23 Aug 2018 10:26:05 +0200
From:   Pierre Morel <pmorel@...ux.ibm.com>
To:     David Hildenbrand <david@...hat.com>,
        Tony Krowiak <akrowiak@...ux.ibm.com>,
        Tony Krowiak <akrowiak@...ux.vnet.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     freude@...ibm.com, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, borntraeger@...ibm.com,
        cohuck@...hat.com, kwankhede@...dia.com,
        bjsdjshi@...ux.vnet.ibm.com, pbonzini@...hat.com,
        alex.williamson@...hat.com, pmorel@...ux.vnet.ibm.com,
        alifm@...ux.vnet.ibm.com, mjrosato@...ux.vnet.ibm.com,
        jjherne@...ux.vnet.ibm.com, thuth@...hat.com,
        pasic@...ux.vnet.ibm.com, berrange@...hat.com,
        fiuczy@...ux.vnet.ibm.com, buendgen@...ibm.com,
        frankja@...ux.ibm.com
Subject: Re: [PATCH v9 21/22] KVM: s390: CPU model support for AP
 virtualization

On 23/08/2018 09:48, David Hildenbrand wrote:
>>>>
>>> I really wonder if we should also export the APXA facility.
>>
>> Given this comment is made within the context of the
>> FACILITIES_KVM_CPUMODEL I might point out that APXA is not
>> indicated by a facilities bit. It is indicated by a bit in
>> the QCI control block returned from the PQAP(QCI)
>> instruction to indicate that APXA is installed on all CPUs.
>>
>>> We can probe and allow that CPU feature. However, we cannot disable it
>>> (as of now).
>>
>> Given this patch series implements passthrough devices,
>> the output of the PQAP(QCI) will always be from a real
>> device - i.e., there will be no way to disable it.
>>
> 
> see below
> 
>>>
>>> We have other CPU features where it is the same case (basically all
>>> subfunctions). See kvm_s390_get_processor_subfunc(). We probe them and
>>> export them, but support to disable them has never been implemented.
>>>
>>> On a high level, we could then e.g. deny to start a QEMU guest if APXA
>>> is available but has been disabled. (until we know that disabling it
>>> actually works - if ever).
>>>
>>> This helps to catch nasty migration bugs (e.g. APXA suddenly
>>> disappearing). Although unlikely, definitely possible.
>>
>> Migration of AP devices is not supported by this patch series, so this
>> should
>> not be an issue.
> 
> Might not be a problem now, but could be later. As I said in a different
> reply, the CPU model in QEMU does not care about KVM.
> 
> I want the QEMU CPU model and the KVM interfaces to be clean and future
> proof. That's why my opinion is to handle PQAP(QCI) just like all the
> other "feature blocks" we already have.
> 

Don't you mix with the TAPQ instruction which has
a T bit to specify interception.
It indeed is not in the subfunction list even it
has a T bit to indicate interception.

TAPQ-t is indicated through the APFT facility.

We can use this bit as an indication of the presence
of APXA, the documentation mention that both are implemented together.

regards,
Pierre


-- 
Pierre Morel
Linux/KVM/QEMU in Böblingen - Germany