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

[Pierre Morel]

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@xxxxxxxxxxxxxxxxxx>
> > > ---
> > [..]
> >
> > > 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));
> > > ÂÂÂÂÂÂÂÂÂ VM_EVENT(kvm, 3, "%s", "DISABLE: DEA keywrapping support");
> > > ÂÂÂÂÂÂÂÂÂ break;
> > > +ÂÂÂ case KVM_S390_VM_CRYPTO_INTERPRET_AP:
> > > +ÂÂÂÂÂÂÂ if (attr->addr) {
> > > +ÂÂÂÂÂÂÂÂÂÂÂ 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:
>
> ÂÂÂ if (!test_kvm_cpu_feat(kvm, KVM_S390_VM_CPU_FEAT_AP))
> ÂÂÂÂÂÂÂ return -EOPNOTSUPP;
>
> ÂÂÂ 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.
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ return -EOPNOTSUPP;

Obviously Halil is speaking on this return statement.
Which returns without unlocking the mutex.



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