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

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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



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