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

[Halil Pasic <pasic@linux.ibm.com>]

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?

> So it is about passing e.g. APXA availability, not QCI itself. (as you
> correctly said, that is stfl 12)
>