Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Reply-To: pmorel@linux.ibm.com
Subject: Re: [PATCH v9 21/22] KVM: s390: CPU model support for AP
 virtualization
To:     David Hildenbrand <david@redhat.com>,
        Tony Krowiak <akrowiak@linux.ibm.com>,
        Tony Krowiak <akrowiak@linux.vnet.ibm.com>,
        linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     freude@de.ibm.com, schwidefsky@de.ibm.com,
        heiko.carstens@de.ibm.com, borntraeger@de.ibm.com,
        cohuck@redhat.com, kwankhede@nvidia.com,
        bjsdjshi@linux.vnet.ibm.com, pbonzini@redhat.com,
        alex.williamson@redhat.com, pmorel@linux.vnet.ibm.com,
        alifm@linux.vnet.ibm.com, mjrosato@linux.vnet.ibm.com,
        jjherne@linux.vnet.ibm.com, thuth@redhat.com,
        pasic@linux.vnet.ibm.com, berrange@redhat.com,
        fiuczy@linux.vnet.ibm.com, buendgen@de.ibm.com,
        frankja@linux.ibm.com
References: <1534196899-16987-1-git-send-email-akrowiak@linux.vnet.ibm.com>
 <1534196899-16987-22-git-send-email-akrowiak@linux.vnet.ibm.com>
 <2c2c4859-ed3e-a492-dd59-78529c7768b2@redhat.com>
 <8f3f3f41-a052-1975-69e2-49e1a662ecab@linux.ibm.com>
 <38b87fc4-9344-70d8-4602-bf8e114769ef@redhat.com>
From:   Pierre Morel <pmorel@linux.ibm.com>
Date:   Thu, 23 Aug 2018 10:26:05 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.9.1
MIME-Version: 1.0
In-Reply-To: <38b87fc4-9344-70d8-4602-bf8e114769ef@redhat.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 8bit
Message-Id: <933db512-fca1-a471-5e2d-b247cfc94fea@linux.ibm.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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