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 12:00 PM, Pierre Morel wrote:
> On 15/03/2018 16:23, Tony Krowiak wrote:
>> On 03/14/2018 05:57 PM, 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?
>> The mutex is unlocked prior to return at the end of the function.
>>>
>>> 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.
>> I think what you're saying is something like this:
>>
>>     if (!test_kvm_cpu_feat(kvm, KVM_S390_VM_CPU_FEAT_AP))
>>         return -EOPNOTSUPP;
>>
>>     kvm->arch.crypto.apie = (attr->addr) ? 1 : 0;
>>
>> I can make arguments for doing this either way, but since the attribute
>> is will most likely only be set by an AP device in userspace, I suppose
>> it makes sense to allow setting of the attribute if the AP feature is
>> installed. It certainly makes sense for the dedicated implementation.
>>>
>>>> +                return -EOPNOTSUPP;
>
> Obviously Halil is speaking on this return statement.
> Which returns without unlocking the mutex.
Got it.
>
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