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


    >>
    >>
    >> Pierre
    >>
    >>
    >>
    >

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

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