linux-kernel - Re: [PATCH v6 05/21] KVM: s390: CPU model support for AP virtualization

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Date:   Mon, 2 Jul 2018 18:20:55 +0200
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Cornelia Huck <cohuck@...hat.com>,
        Tony Krowiak <akrowiak@...ux.ibm.com>
Cc:     Christian Borntraeger <borntraeger@...ibm.com>,
        Tony Krowiak <akrowiak@...ux.vnet.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, freude@...ibm.com, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.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
Subject: Re: [PATCH v6 05/21] KVM: s390: CPU model support for AP
 virtualization



On 07/02/2018 06:11 PM, Cornelia Huck wrote:
> On Mon, 2 Jul 2018 11:54:28 -0400
> Tony Krowiak <akrowiak@...ux.ibm.com> wrote:
> 
>> On 07/02/2018 11:41 AM, Cornelia Huck wrote:
>>> On Mon, 2 Jul 2018 11:37:11 -0400
>>> Tony Krowiak <akrowiak@...ux.ibm.com> wrote:
>>>   
>>>> On 07/02/2018 10:38 AM, Christian Borntraeger wrote:
>>>>> On 06/29/2018 11:11 PM, Tony Krowiak wrote:
>>>>>> Introduces a new CPU model feature and two CPU model
>>>>>> facilities to support AP virtualization for KVM guests.
>>>>>>
>>>>>> CPU model feature:
>>>>>>
>>>>>> The KVM_S390_VM_CPU_FEAT_AP feature indicates that
>>>>>> AP instructions are available on the guest. This
>>>>>> feature will be enabled by the kernel only if the AP
>>>>>> instructions are installed on the linux host. This feature
>>>>>> must be specifically turned on for the KVM guest from
>>>>>> userspace to use the VFIO AP device driver for guest
>>>>>> access to AP devices.
>>>>>>
>>>>>> CPU model facilities:
>>>>>>
>>>>>> 1. AP Query Configuration Information (QCI) facility is installed.
>>>>>>
>>>>>>       This is indicated by setting facilities bit 12 for
>>>>>>       the guest. The kernel will not enable this facility
>>>>>>       for the guest if it is not set on the host. This facility
>>>>>>       must not be set by userspace if the KVM_S390_VM_CPU_FEAT_AP
>>>>>>       feature is not installed.
>>>>>>
>>>>>>       If this facility is not set for the KVM guest, then only
>>>>>>       APQNs with an APQI less than 16 will be available to the
>>>>>>       guest regardless of the guest's matrix configuration. This
>>>>>>       is a limitation of the AP bus running on the guest.
>>>>>>
>>>>>> 2. AP Facilities Test facility (APFT) is installed.
>>>>>>
>>>>>>       This is indicated by setting facilities bit 15 for
>>>>>>       the guest. The kernel will not enable this facility for
>>>>>>       the guest if it is not set on the host. This facility
>>>>>>       must not be set by userspace if the KVM_S390_VM_CPU_FEAT_AP
>>>>>>       feature is not installed.
>>>>>>
>>>>>>       If this facility is not set for the KVM guest, then no
>>>>>>       AP devices will be available to the guest regardless of
>>>>>>       the guest's matrix configuration. This is a limitation
>>>>>>       of the AP bus running under the guest.
>>>>>>
>>>>>> Reviewed-by: Christian Borntraeger <borntraeger@...ibm.com>
>>>>>> Reviewed-by: Halil Pasic <pasic@...ux.ibm.com>
>>>>>> Signed-off-by: Tony Krowiak <akrowiak@...ux.ibm.com>
>>>>> I think it probably should be at the end of the series, other than that its good.
>>>> If I move this to the end of the series, the very next patch checks the
>>>>
>>>> KVM_S390_VM_CPU_FEAT_AP feature?
>>> Introduce it here, offer it only with the last patch?
>>
>> I apologize, but I don't know what you mean by this. Are you suggesting
>> this patch
>> should only include the #define for KVM_S390_VM_CPU_FEAT_AP?
> 
> Yes, just introduce the definition here (so code later in the series
> can refer to it) and flip the switch (offer the bit) as the final
> patch.
> 

The other features introduced and exposed here are no different. For
KVM_S390_VM_CPU_FEAT_AP defer exposing means defer allow_cpu_feat();
for the STFLE features, defer adding to FACILITIES_KVM_CPUMODEL.

Anyway, I think the definition should be squashed into #6. Expose the
features after patch #6 is in place or expose them at the end of the
series is IMHO a matter of taste -- and I lean towards expose at the
end of the series.

Regards,
Halil