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

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Message-Id: <3afd28d7-57c6-cda6-a436-18da32ef0df3@linux.vnet.ibm.com>
Date:   Thu, 15 Mar 2018 11:23:15 -0400
From:   Tony Krowiak <akrowiak@...ux.vnet.ibm.com>
To:     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, pmorel@...ux.vnet.ibm.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 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;
>> +			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.
Assuming the scenario plays out as you described, why would the other vcpus
be using the old ECA.28 value if the kvm_s390_vcpu_crypto_setup() function
is executed for each of them to set the new value for 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?
Unless I am missing something in the scenario you described, it seems that
the reason the exit_sie(vcpu) function is called is to ensure that the vcpus
that are already running acquire the new attribute values changed by this
function when they are restored to SIE. Of course, my assumption is that
the kvm_arch_vcpu_setup() function - which calls the 
kvm_s390_vcpu_crypto_setup()
function - is invoked when the vcpu is restored to SIE.
>
> Regards,
> Halil
>
> [..]