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

[Pierre Morel]

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@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?
> > >
> > > 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.
> > >
> > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ return -EOPNOTSUPP;
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ kvm->arch.crypto.apie = 1;
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ VM_EVENT(kvm, 3, "%s",
> > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ "ENABLE: AP interpretive execution");
> > > > +ÂÂÂÂÂÂÂ } else {
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ kvm->arch.crypto.apie = 0;
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ VM_EVENT(kvm, 3, "%s",
> > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ "DISABLE: AP interpretive execution");
> > > > +ÂÂÂÂÂÂÂ }
> > > > +ÂÂÂÂÂÂÂ break;
> > > > ÂÂÂÂÂ default:
> > > > ÂÂÂÂÂÂÂÂÂ mutex_unlock(&kvm->lock);
> > > > ÂÂÂÂÂÂÂÂÂ return -ENXIO;
> > > I wonder how the loop after this switch works for 
> > > KVM_S390_VM_CRYPTO_INTERPRET_AP:
> > >
> > > ÂÂÂÂÂÂÂÂ kvm_for_each_vcpu(i, vcpu, kvm) {
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ kvm_s390_vcpu_crypto_setup(vcpu);
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ exit_sie(vcpu);
> > > ÂÂÂÂÂÂÂÂ }
> > >
> > > ÂFrom not doing something like for KVM_S390_VM_CRYPTO_INTERPRET_AP
> > >
> > > ÂÂÂÂÂÂÂÂ if (kvm->created_vcpus) {
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ mutex_unlock(&kvm->lock);
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ 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 BÃblingen - Germany