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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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From:   Pierre Morel <pmorel@linux.vnet.ibm.com>
Date:   Fri, 16 Mar 2018 08:51:40 +0100
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On 16/03/2018 00:39, Tony Krowiak wrote:
> On 03/15/2018 01:56 PM, Pierre Morel wrote:
>> On 15/03/2018 18:21, Tony Krowiak wrote:
>>> 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(). 
>>
>> hum, sorry for this.
>> However, the idea pertains, not to call this function from inside an 
>> ioctl changing crypto parameters, but only during vcpu creation.
> Unfortunately, the ioctl does not get called until after the vcpus are 
> created (see my comments below)

That is why I think you should not change the ECA field from the crypto 
ioctl but only during the vcpu initialization phase.

>>
>>
>>
>>> 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
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>

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