Re: [PATCH v3 04/14] KVM: s390: device attribute to set AP interpretive execution

[Tony Krowiak <akrowiak@linux.vnet.ibm.com>]

On 03/16/2018 03:51 AM, Pierre Morel wrote:
> 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.
By what means do you suggest we do that?
>
>
>>>
>>>
>>>
>>>> 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
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>