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Subject: Re: [PATCH v9 21/22] KVM: s390: CPU model support for AP
 virtualization
From:   David Hildenbrand <david@redhat.com>
To:     Tony Krowiak <akrowiak@linux.vnet.ibm.com>,
        linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     freude@de.ibm.com, schwidefsky@de.ibm.com,
        heiko.carstens@de.ibm.com, borntraeger@de.ibm.com,
        cohuck@redhat.com, kwankhede@nvidia.com,
        bjsdjshi@linux.vnet.ibm.com, pbonzini@redhat.com,
        alex.williamson@redhat.com, pmorel@linux.vnet.ibm.com,
        alifm@linux.vnet.ibm.com, mjrosato@linux.vnet.ibm.com,
        jjherne@linux.vnet.ibm.com, thuth@redhat.com,
        pasic@linux.vnet.ibm.com, berrange@redhat.com,
        fiuczy@linux.vnet.ibm.com, buendgen@de.ibm.com,
        frankja@linux.ibm.com, Tony Krowiak <akrowiak@linux.ibm.com>
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Organization: Red Hat GmbH
Message-ID: <95e6ee74-69aa-9805-3233-b9ec81fce516@redhat.com>
Date:   Wed, 22 Aug 2018 13:24:56 +0200
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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)

That would imply that there is actually theoretically a way to fake away
certain AP facilities.

-- 

Thanks,

David / dhildenb