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

[David Hildenbrand]

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@xxxxxxxxxxxxx>
>>>>>>
>>>>>> 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@xxxxxxxxxxxxx>
>>>>>> Reviewed-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
>>>>>> Reviewed-by: Halil Pasic <pasic@xxxxxxxxxxxxx>
>>>>>> Tested-by: Michael Mueller <mimu@xxxxxxxxxxxxx>
>>>>>> Tested-by: Farhan Ali <alifm@xxxxxxxxxxxxx>
>>>>>> Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
>>>>>> ---
>>>>>>    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.

-- 

Thanks,

David / dhildenb