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

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