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

David Hildenbrand
2018-08-23T11:12:54Z

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.

>
> 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.

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

>
> Regards,
> Pierre
>

--

Thanks,

David / dhildenb