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    SubjectRe: [PATCH v3 04/14] KVM: s390: device attribute to set AP interpretive execution
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    On 16/03/2018 00:39, Tony Krowiak wrote:
    > 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)

    That is why I think you should not change the ECA field from the crypto
    ioctl but only during the vcpu initialization phase.

    >>
    >>
    >>
    >>> 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
    >>>>>>
    >>>>>>
    >>>>>>
    >>>>>
    >>>>
    >>>
    >>
    >

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

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