Re: [PATCH v7 1/1] s390x: KVM: guest support for topology function

[Janosch Frank]

On 2/18/22 19:24, Pierre Morel wrote:
> On 2/18/22 18:27, Pierre Morel wrote:
> > On 2/18/22 15:28, Janosch Frank wrote:
> > > On 2/18/22 14:13, Pierre Morel wrote:
> > > > On 2/17/22 18:17, Nico Boehr wrote:
> > > > > On Thu, 2022-02-17 at 10:59 +0100, Pierre Morel wrote:
> > > > > [...]
> > > > > > diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> > > > > > index 2296b1ff1e02..af7ea8488fa2 100644
> > > > > > --- a/arch/s390/kvm/kvm-s390.c
> > > > > > +++ b/arch/s390/kvm/kvm-s390.c
> > > > > [...]
> > >
> > > Why is there no interface to clear the SCA_UTILITY_MTCR on a subsystem
> > > reset?
> >
> > Right, I had one in my first version based on interception but I forgot
> > to implement an equivalent for KVM as I modified the implementation for
> > interpretation.
> > I will add this.
> >
> > > > > > -void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> > > > > > +/**
> > > > > > + * kvm_s390_vcpu_set_mtcr
> > > > > > + * @vcp: the virtual CPU
> > > > > > + *
> > > > > > + * Is only relevant if the topology facility is present.
> > > > > > + *
> > > > > > + * Updates the Multiprocessor Topology-Change-Report to signal
> > > > > > + * the guest with a topology change.
> > > > > > + */
> > > > > > +static void kvm_s390_vcpu_set_mtcr(struct kvm_vcpu *vcpu)
> > > > > >     {
> > > > > > +       struct esca_block *esca = vcpu->kvm->arch.sca;
> > > > >
> > > > > utility is at the same offset for the bsca and the esca, still
> > > > > wondering whether it is a good idea to assume esca here...
> > > >
> > > > We can take bsca to be coherent with the include file where we define
> > > > ESCA_UTILITY_MTCR inside the bsca.
> > > > And we can rename the define to SCA_UTILITY_MTCR as it is common for
> > > > both BSCA and ESCA the (E) is too much.
> > >
> > > Yes and maybe add a comment that it's at the same offset for esca so
> > > there won't come up further questions in the future.
> >
> > OK
> >
> > > > > [...]
> > > > > > diff --git a/arch/s390/kvm/kvm-s390.h b/arch/s390/kvm/kvm-s390.h
> > > > > > index 098831e815e6..af04ffbfd587 100644
> > > > > > --- a/arch/s390/kvm/kvm-s390.h
> > > > > > +++ b/arch/s390/kvm/kvm-s390.h
> > > > > > @@ -503,4 +503,29 @@ void kvm_s390_vcpu_crypto_reset_all(struct kvm
> > > > > > *kvm);
> > > > > >      */
> > > > > >     extern unsigned int diag9c_forwarding_hz;
> > > > > > +#define S390_KVM_TOPOLOGY_NEW_CPU -1
> > > > > > +/**
> > > > > > + * kvm_s390_topology_changed
> > > > > > + * @vcpu: the virtual CPU
> > > > > > + *
> > > > > > + * If the topology facility is present, checks if the CPU toplogy
> > > > > > + * viewed by the guest changed due to load balancing or CPU hotplug.
> > > > > > + */
> > > > > > +static inline bool kvm_s390_topology_changed(struct kvm_vcpu *vcpu)
> > > > > > +{
> > > > > > +       if (!test_kvm_facility(vcpu->kvm, 11))
> > > > > > +               return false;
> > > > > > +
> > > > > > +       /* A new vCPU has been hotplugged */
> > > > > > +       if (vcpu->arch.prev_cpu == S390_KVM_TOPOLOGY_NEW_CPU)
> > > > > > +               return true;
> > > > > > +
> > > > > > +       /* The real CPU backing up the vCPU moved to another socket
> > > > > > */
> > > > > > +       if (topology_physical_package_id(vcpu->cpu) !=
> > > > > > +           topology_physical_package_id(vcpu->arch.prev_cpu))
> > > > > > +               return true;
> > > > >
> > > > > Why is it OK to look just at the physical package ID here? What if the
> > > > > vcpu for example moves to a different book, which has a core with the
> > > > > same physical package ID?
> > >
> > > I'll need to look up stsi 15* output to understand this.
> > > But the architecture states that any change to the stsi 15 output sets
> > > the change bit so I'd guess Nico is correct.
> >
> > Yes, Nico is correct, as I already answered, however it is not any
> > change of stsi(15) but a change of stsi(15.1.2) output which sets the
> > change bit.
>
> hum, that is what the POP says but in fact you are right a change of
> topology that changes the output of any STSI(15) sets the topology
> change report bit as the output of STSI(15.1.2) would be changed too
> obviously.

In this case I was just being too lazy to look up the correct query code 
but I knew it started with fc 15. It was Friday after all :-)

> Regards,
> Pierre