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

[David Hildenbrand]

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.

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

-- 

Thanks,

David / dhildenb