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

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

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)
>
>
>
>> 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
>>>>>
>>>>>
>>>>>
>>>>
>>>
>>
>