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Subject: Re: [PATCH v3 04/14] KVM: s390: device attribute to set AP
 interpretive execution
To:     Pierre Morel <pmorel@linux.vnet.ibm.com>,
        Halil Pasic <pasic@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, alifm@linux.vnet.ibm.com,
        mjrosato@linux.vnet.ibm.com, jjherne@linux.vnet.ibm.com,
        thuth@redhat.com, berrange@redhat.com, fiuczy@linux.vnet.ibm.com,
        buendgen@de.ibm.com
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 <d86ad01e-1c79-c14c-b4da-09d9a31572bd@linux.vnet.ibm.com>
 <21bd029b-3500-3461-ce98-68ad3ae9b647@linux.vnet.ibm.com>
 <46a7e838-2be2-9587-6eb2-3bba95485609@linux.vnet.ibm.com>
 <5ed8017b-0168-9a50-234b-cfe9258eab72@linux.vnet.ibm.com>
From:   Tony Krowiak <akrowiak@linux.vnet.ibm.com>
Date:   Thu, 15 Mar 2018 13:21:54 -0400
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On 03/15/2018 11:45 AM, Pierre Morel wrote:
> On 15/03/2018 16:26, Tony Krowiak wrote:
>> On 03/15/2018 09:00 AM, Pierre Morel wrote:
>>> On 14/03/2018 22:57, Halil Pasic wrote:
>>>>
>>>> On 03/14/2018 07:25 PM, Tony Krowiak wrote:
>>>>> The VFIO AP device model exploits interpretive execution of AP
>>>>> instructions (APIE) to provide guests passthrough access to AP
>>>>> devices. This patch introduces a new device attribute in the
>>>>> KVM_S390_VM_CRYPTO device attribute group to set APIE from
>>>>> the VFIO AP device defined on the guest.
>>>>>
>>>>> Signed-off-by: Tony Krowiak <akrowiak@linux.vnet.ibm.com>
>>>>> ---
>>>> [..]
>>>>
>>>>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>>>>> index a60c45b..bc46b67 100644
>>>>> --- a/arch/s390/kvm/kvm-s390.c
>>>>> +++ b/arch/s390/kvm/kvm-s390.c
>>>>> @@ -815,6 +815,19 @@ static int kvm_s390_vm_set_crypto(struct kvm 
>>>>> *kvm, struct kvm_device_attr *attr)
>>>>> sizeof(kvm->arch.crypto.crycb->dea_wrapping_key_mask));
>>>>>           VM_EVENT(kvm, 3, "%s", "DISABLE: DEA keywrapping support");
>>>>>           break;
>>>>> +    case KVM_S390_VM_CRYPTO_INTERPRET_AP:
>>>>> +        if (attr->addr) {
>>>>> +            if (!test_kvm_cpu_feat(kvm, KVM_S390_VM_CPU_FEAT_AP))
>>>> Unlock mutex before returning?
>>>>
>>>> Maybe flip conditions (don't allow manipulating apie if feature not 
>>>> there).
>>>> Clearing the anyways clear apie if feature not there ain't too bad, 
>>>> but
>>>> rejecting the operation appears nicer to me.
>>>>
>>>>> +                return -EOPNOTSUPP;
>>>>> +            kvm->arch.crypto.apie = 1;
>>>>> +            VM_EVENT(kvm, 3, "%s",
>>>>> +                 "ENABLE: AP interpretive execution");
>>>>> +        } else {
>>>>> +            kvm->arch.crypto.apie = 0;
>>>>> +            VM_EVENT(kvm, 3, "%s",
>>>>> +                 "DISABLE: AP interpretive execution");
>>>>> +        }
>>>>> +        break;
>>>>>       default:
>>>>>           mutex_unlock(&kvm->lock);
>>>>>           return -ENXIO;
>>>> I wonder how the loop after this switch works for 
>>>> KVM_S390_VM_CRYPTO_INTERPRET_AP:
>>>>
>>>>          kvm_for_each_vcpu(i, vcpu, kvm) {
>>>>                  kvm_s390_vcpu_crypto_setup(vcpu);
>>>>                  exit_sie(vcpu);
>>>>          }
>>>>
>>>>  From not doing something like for KVM_S390_VM_CRYPTO_INTERPRET_AP
>>>>
>>>>          if (kvm->created_vcpus) {
>>>>                  mutex_unlock(&kvm->lock);
>>>>                  return -EBUSY;
>>>> and from the aforementioned loop I guess ECA.28 can be changed
>>>> for a running guest.
>>>>
>>>> If there are running vcpus when KVM_S390_VM_CRYPTO_INTERPRET_AP is
>>>> changed (set) these will be taken out of SIE by exit_sie(). Then 
>>>> for the
>>>> corresponding threads the control probably goes to QEMU (the 
>>>> emulator in
>>>> the userspace). And it puts that vcpu back into the SIE, and then that
>>>> cpu starts acting according to the new ECA.28 value.  While other 
>>>> vcpus
>>>> may still work with the old value of ECA.28.
>>>>
>>>> I'm not saying what I describe above is necessarily something broken.
>>>> But I would like to have it explained, why is it OK -- provided I 
>>>> did not
>>>> make any errors in my reasoning (assumptions included).
>>>>
>>>> Can you help me understand this code?
>>>>
>>>> Regards,
>>>> Halil
>>>>
>>>> [..]
>>>>
>>>
>>> I have the same concerns as Halil.
>>>
>>> We do not need to change the virtulization type
>>> (hardware/software) on the fly for the current use case.
>>>
>>> Couldn't we delay this until we have one and in between only make 
>>> the vCPU hotplug clean?
>>>
>>> We only need to let the door open for the day we have such a use case.
>> Are you suggesting this code be removed? If so, then where and under 
>> what conditions would
>> you suggest setting ECA.28 given you objected to setting it based on 
>> whether the
>> AP feature is installed?
>
> I would only call kvm_s390_vcpu_crypto_setup() from inside 
> kvm_arch_vcpu_init()
> as it is already.
It is not called from kvm_arch_vcpu_init(), it is called from 
kvm_arch_vcpu_setup(). Also,
this loop was already here, I did not put it in. Assuming whomever put 
it there did so
for a reason, it is not my place to remove it. According to a trace I 
ran, the calls to this
function occur after the vcpus are created. Consequently, the 
kvm_s390_vcpu_crypto_setup()
function would not be called without the loop and neither the key 
wrapping support nor the
ECA_APIE would be configured in the vcpu's SIE descriptor.

If you have a better idea for where/how to set this flag, I'm all
ears. It would be nice if it could be set before the vcpus are created, 
but I haven't
found a good candidate. I suspect that the loop was put in to make sure 
that all vcpus
get updated regardless of whether they are running or not, but I don't 
know what happens
after a vcpu is kicked out of SIE. I suspect, as Halil surmised, that QEMU
restores the vcpus to SIE. This would seemingly cause the 
kvm_arch_vcpu_setup() to get
called at which time the ECA_APIE value as well as the key wrapping 
values will get set.
If somebody has knowledge of the flow here, please feel free to pitch in.
>
>
>
>>>
>>>
>>> Pierre
>>>
>>>
>>>
>>
>