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

[Pierre Morel <pmorel@xxxxxxxxxxxxx>]

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
> [...]
> >   
> > -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.

> [...]
> > 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?

You are right, we should look at the drawer and book id too.
Something like that I think:

        if ((topology_physical_package_id(vcpu->cpu) !=
             topology_physical_package_id(vcpu->arch.prev_cpu)) ||
            (topology_book_id(vcpu->cpu) !=
             topology_book_id(vcpu->arch.prev_cpu)) ||
            (topology_drawer_id(vcpu->cpu) !=
             topology_drawer_id(vcpu->arch.prev_cpu)))
                return true;


Thanks,
regards,
Pierre
--
Pierre Morel
IBM Lab Boeblingen