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

[Nico Boehr]

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... 

[...]
> 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?