linux-kernel - Re: [PATCH v2 04/15] KVM: s390: CPU model support for AP virtualization

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Date:   Wed, 28 Feb 2018 12:58:42 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>,
        Tony Krowiak <akrowiak@...ux.vnet.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     freude@...ibm.com, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, cohuck@...hat.com, kwankhede@...dia.com,
        bjsdjshi@...ux.vnet.ibm.com, pbonzini@...hat.com,
        alex.williamson@...hat.com, pmorel@...ux.vnet.ibm.com,
        alifm@...ux.vnet.ibm.com, mjrosato@...ux.vnet.ibm.com,
        jjherne@...ux.vnet.ibm.com, thuth@...hat.com,
        pasic@...ux.vnet.ibm.com, fiuczy@...ux.vnet.ibm.com,
        buendgen@...ibm.com
Subject: Re: [PATCH v2 04/15] KVM: s390: CPU model support for AP
 virtualization

On 28.02.2018 12:40, Christian Borntraeger wrote:
> 
> 
> On 02/28/2018 10:48 AM, David Hildenbrand wrote:
>> On 27.02.2018 15:28, Tony Krowiak wrote:
>>> 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 the
>>> AP facilities are installed on the KVM guest. This
>>> feature will be enabled by the kernel only if the AP
>>> facilities are installed on the linux host. This feature
>>> must be specifically turned on for the KVM guest from
>>> userspace to allow guest access to AP devices installed
>>> on the linux host.
>>>
>>> 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. This facility
>>>    must not be set by userspace if the KVM_S390_VM_CPU_FEAT_AP
>>>    feature is not installed.
>>>
>>> 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. This facility
>>>    must not be set by userspace if the KVM_S390_VM_CPU_FEAT_AP
>>>    feature is not installed.
>>>
>>> Reviewed-by: Christian Borntraeger <borntraeger@...ibm.com>
>>> Reviewed-by: Halil Pasic <pasic@...ux.vnet.ibm.com>
>>> Signed-off-by: Tony Krowiak <akrowiak@...ux.vnet.ibm.com>
>>> ---
>>>  arch/s390/include/uapi/asm/kvm.h |    1 +
>>>  arch/s390/kvm/kvm-s390.c         |    4 ++++
>>>  arch/s390/tools/gen_facilities.c |    2 ++
>>>  3 files changed, 7 insertions(+), 0 deletions(-)
>>>
>>> diff --git a/arch/s390/include/uapi/asm/kvm.h b/arch/s390/include/uapi/asm/kvm.h
>>> index 4cdaa55..a580dec 100644
>>> --- a/arch/s390/include/uapi/asm/kvm.h
>>> +++ b/arch/s390/include/uapi/asm/kvm.h
>>> @@ -130,6 +130,7 @@ struct kvm_s390_vm_cpu_machine {
>>>  #define KVM_S390_VM_CPU_FEAT_PFMFI	11
>>>  #define KVM_S390_VM_CPU_FEAT_SIGPIF	12
>>>  #define KVM_S390_VM_CPU_FEAT_KSS	13
>>> +#define KVM_S390_VM_CPU_FEAT_AP		14
>>>  struct kvm_s390_vm_cpu_feat {
>>>  	__u64 feat[16];
>>>  };
>>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>>> index de1e299..c68ca86 100644
>>> --- a/arch/s390/kvm/kvm-s390.c
>>> +++ b/arch/s390/kvm/kvm-s390.c
>>> @@ -347,6 +347,10 @@ static void kvm_s390_cpu_feat_init(void)
>>>  
>>>  	if (MACHINE_HAS_ESOP)
>>>  		allow_cpu_feat(KVM_S390_VM_CPU_FEAT_ESOP);
>>> +
>>> +	if (ap_instructions_installed()) /* AP instructions installed on host */
>>> +		allow_cpu_feat(KVM_S390_VM_CPU_FEAT_AP);
>>
>> Don't we have a SIE specific AP feature? So is it true, that once we
>> have AP instructions, we are allowed to use them for SIE? Isn't there a
>> "AP interpretation facility" or anything like that? (that unlocks ECA_APIE)
> 
> This seems to be coupled to the AP facility and there is no facility or scp bit.
> AP is too old to have that (predates STFLE)
> 

Interesting, so we (I :) ) didn't implement vSIE support back then
because we never indicated the AP facility (sensed by executing the
instruction) to the guest.

So ECA_APIE can be used when we sense the AP facility. Thanks!

-- 

Thanks,

David / dhildenb