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

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Date:   Thu, 15 Mar 2018 17:00:03 +0100
From:   Pierre Morel <pmorel@...ux.vnet.ibm.com>
To:     Tony Krowiak <akrowiak@...ux.vnet.ibm.com>,
        Halil Pasic <pasic@...ux.vnet.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     freude@...ibm.com, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, borntraeger@...ibm.com,
        cohuck@...hat.com, kwankhede@...dia.com,
        bjsdjshi@...ux.vnet.ibm.com, pbonzini@...hat.com,
        alex.williamson@...hat.com, alifm@...ux.vnet.ibm.com,
        mjrosato@...ux.vnet.ibm.com, jjherne@...ux.vnet.ibm.com,
        thuth@...hat.com, berrange@...hat.com, fiuczy@...ux.vnet.ibm.com,
        buendgen@...ibm.com
Subject: Re: [PATCH v3 04/14] KVM: s390: device attribute to set AP
 interpretive execution

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@...ux.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.



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