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Subject: Re: [PATCH v3 04/14] KVM: s390: device attribute to set AP
 interpretive execution
To:     Tony Krowiak <akrowiak@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
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        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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 <46a7e838-2be2-9587-6eb2-3bba95485609@linux.vnet.ibm.com>
From:   Pierre Morel <pmorel@linux.vnet.ibm.com>
Date:   Thu, 15 Mar 2018 16:45:35 +0100
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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.


>>
>>
>> Pierre
>>
>>
>>
>

-- 
Pierre Morel
Linux/KVM/QEMU in Böblingen - Germany