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Subject: Re: [PATCH v9 21/22] KVM: s390: CPU model support for AP
 virtualization
To:     David Hildenbrand <david@redhat.com>,
        Halil Pasic <pasic@linux.ibm.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
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From:   Pierre Morel <pmorel@linux.ibm.com>
Date:   Thu, 23 Aug 2018 14:47:39 +0200
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On 23/08/2018 13:12, David Hildenbrand wrote:
> On 23.08.2018 13:10, Pierre Morel wrote:
>> On 23/08/2018 12:28, David Hildenbrand wrote:
>>> On 23.08.2018 12:00, Halil Pasic wrote:
>>>>
>>>>
>>>> On 08/23/2018 09:44 AM, David Hildenbrand wrote:
>>>>> On 22.08.2018 22:16, Tony Krowiak wrote:
>>>>>> On 08/22/2018 07:24 AM, David Hildenbrand wrote:
>>>>>>> On 22.08.2018 13:19, David Hildenbrand wrote:
>>>>>>>> On 13.08.2018 23:48, Tony Krowiak wrote:
>>>>>>>>> From: Tony Krowiak <akrowiak@linux.ibm.com>
>>>>>>>>>
>>>>>>>>> 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.
>>>>>>>>>
>>>>>>>>>        If this facility is not set for the KVM guest, then only
>>>>>>>>>        APQNs with an APQI less than 16 will be used by a Linux
>>>>>>>>>        guest regardless of the matrix configuration for the virtual
>>>>>>>>>        machine. This is a limitation of the Linux AP bus.
>>>>>>>>>
>>>>>>>>> 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.
>>>>>>>>>
>>>>>>>>>        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 for the virtual
>>>>>>>>>        machine. This is a limitation of the Linux AP bus.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
>>>>>>>>> Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
>>>>>>>>> Reviewed-by: Halil Pasic <pasic@linux.ibm.com>
>>>>>>>>> Tested-by: Michael Mueller <mimu@linux.ibm.com>
>>>>>>>>> Tested-by: Farhan Ali <alifm@linux.ibm.com>
>>>>>>>>> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
>>>>>>>>> ---
>>>>>>>>>      arch/s390/kvm/kvm-s390.c         |    5 +++++
>>>>>>>>>      arch/s390/tools/gen_facilities.c |    2 ++
>>>>>>>>>      2 files changed, 7 insertions(+), 0 deletions(-)
>>>>>>>>>
>>>>>>>>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>>>>>>>>> index 1e8cb67..d5e04d2 100644
>>>>>>>>> --- a/arch/s390/kvm/kvm-s390.c
>>>>>>>>> +++ b/arch/s390/kvm/kvm-s390.c
>>>>>>>>> @@ -367,6 +367,11 @@ static void kvm_s390_cpu_feat_init(void)
>>>>>>>>>      
>>>>>>>>>      	if (MACHINE_HAS_ESOP)
>>>>>>>>>      		allow_cpu_feat(KVM_S390_VM_CPU_FEAT_ESOP);
>>>>>>>>> +
>>>>>>>>> +	/* Check if AP instructions installed on host */
>>>>>>>>> +	if (ap_instructions_available())
>>>>>>>>> +		allow_cpu_feat(KVM_S390_VM_CPU_FEAT_AP);
>>>>>>>>> +
>>>>>>>>>      	/*
>>>>>>>>>      	 * We need SIE support, ESOP (PROT_READ protection for gmap_shadow),
>>>>>>>>>      	 * 64bit SCAO (SCA passthrough) and IDTE (for gmap_shadow unshadowing).
>>>>>>>>> diff --git a/arch/s390/tools/gen_facilities.c b/arch/s390/tools/gen_facilities.c
>>>>>>>>> index 90a8c9e..a52290b 100644
>>>>>>>>> --- a/arch/s390/tools/gen_facilities.c
>>>>>>>>> +++ b/arch/s390/tools/gen_facilities.c
>>>>>>>>> @@ -106,6 +106,8 @@ struct facility_def {
>>>>>>>>>      
>>>>>>>>>      		.name = "FACILITIES_KVM_CPUMODEL",
>>>>>>>>>      		.bits = (int[]){
>>>>>>>>> +			12, /* AP Query Configuration Information */
>>>>>>>>> +			15, /* AP Facilities Test */
>>>>>>>>>      			-1  /* END */
>>>>>>>>>      		}
>>>>>>>>>      	},
>>>>>>>>>
>>>>>>>> I really wonder if we should also export the APXA facility.
>>>>>>>>
>>>>>>>> We can probe and allow that CPU feature. However, we cannot disable it
>>>>>>>> (as of now).
>>>>>>>>
>>>>>>>> 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.
>>>>>>>>
>>>>>>>>
>>>>>>>> Are there any other AP related facilities that the guest can from now on
>>>>>>>> probe that should also become part of the CPU model?
>>>>>>>>
>>>>>>> To be more precise, shouldn't PQAP(QCI) be handled just like other
>>>>>>> subfunctions? (I remember it should)
>>>>>>
>>>>>> When you suggest PQAP(QCI) be handled like other subfunctions, are you
>>>>>> suggesting that there should be a field in struct kvm_s390_vm_cpu_subfunc
>>>>>> with a bit indicating the QCI subfunction is available? The availability
>>>>>> of the QCI subfunction of the PQAP instruction is determined by facilities
>>>>>> bit 12. Is it not enough to export facilities bit 12?
>>>>>
>>>>> The feature block (128 bit) from PQAP(QCI) should be passed through a
>>>>> subfunction block to QEMU.
>>>>>
>>>>
>>>> I'm confused, which 128 bit?
>>>
>>>
>>> Me too :) , I was assuming this block to be 128bit, but the qci block
>>> has 128 bytes....
>>>
>>> And looking at arch/s390/include/asm/ap.h, there is a lot of information
>>> contained that is definitely not of interest for CPU models...
>>>
>>> I wonder if there is somewhere defined which bits are reserved for
>>> future features/facilities, compared to ap masks and such.
>>>
>>> This is really hard to understand/plan without access to documentation.
>>>
>>> You (Halil, Tony, Pier, ...) should have a look if what I described
>>> related to PQAP(QCI) containing features that should get part of the CPU
>>> model makes sense or not. For now I was thinking that there is some part
>>> inside of QCI that is strictly reserved for facilities/features that we
>>> can use.
>>>
>>
>> David,
>> I already answered to you on this subject.
>>
>> First,
>> Are you sure you do not mistake QCI for TAPQ which has the t bit
>> instruction interception bit as all the instructions you use as
>> subfunctions?
> 
> Yes, I am pretty sure it is PQAP(QCI), please check with Christian /
> architecture documentations.

OK.

> 
>>
>> Second,
>> The TAPQ interception bit is exposed through the facility bit 15
>> and is documented as being installed when the APXA facility is installed.
>>
>> If we have the APFT, we have the APXA, problem seems solved to me.

hum. wrong, sorry, the assertion is in the wrong way...

> 
> What is apsc, qact, rc8a in the qci blocks? are the facility bits?

Yes, facility bits concerning the AP instructions

> 
>>
>> Regards,
>> Pierre
>>
> 
> 


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