Re: [PATCH v10 3/3] KVM: s390: resetting the Topology-Change-Report

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On 6/24/22 08:50, Janosch Frank wrote:
On 6/20/22 14:54, Pierre Morel wrote:
During a subsystem reset the Topology-Change-Report is cleared.
Let's give userland the possibility to clear the MTCR in the case
of a subsystem reset.

To migrate the MTCR, we give userland the possibility to
query the MTCR state.

We indicate KVM support for the CPU topology facility with a new
KVM capability: KVM_CAP_S390_CPU_TOPOLOGY.

Signed-off-by: Pierre Morel <pmorel@xxxxxxxxxxxxx>
---
  Documentation/virt/kvm/api.rst   | 31 +++++++++++
  arch/s390/include/uapi/asm/kvm.h | 10 ++++
  arch/s390/kvm/kvm-s390.c         | 96 ++++++++++++++++++++++++++++++++
  include/uapi/linux/kvm.h         |  1 +
  4 files changed, 138 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 11e00a46c610..326f8b7e7671 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -7956,6 +7956,37 @@ should adjust CPUID leaf 0xA to reflect that the PMU is disabled.
  When enabled, KVM will exit to userspace with KVM_EXIT_SYSTEM_EVENT of
  type KVM_SYSTEM_EVENT_SUSPEND to process the guest suspend request.
+8.37 KVM_CAP_S390_CPU_TOPOLOGY
+------------------------------
+
+:Capability: KVM_CAP_S390_CPU_TOPOLOGY
+:Architectures: s390
+:Type: vm
+
+This capability indicates that KVM will provide the S390 CPU Topology
+facility which consist of the interpretation of the PTF instruction for
+the Function Code 2 along with interception and forwarding of both the

Making function code capital surprises me when reading.

wanted to highlight FC.
I remove it.


+PTF instruction with Function Codes 0 or 1 and the STSI(15,1,x)
+instruction to the userland hypervisor.
+
+The stfle facility 11, CPU Topology facility, should not be provided

s/provided/indicated

OK

+to the guest without this capability.
+
+When this capability is present, KVM provides a new attribute group
+on vm fd, KVM_S390_VM_CPU_TOPOLOGY.
+This new attribute allows to get, set or clear the Modified Change
+Topology Report (MTCR) bit of the SCA through the kvm_device_attr
+structure.
+
+Getting the MTCR bit is realized by using a kvm_device_attr attr
+entry value of KVM_GET_DEVICE_ATTR and with kvm_device_attr addr
+entry pointing to the address of a struct kvm_cpu_topology.
+The value of the MTCR is return by the bit mtcr of the structure. > +
+When using KVM_SET_DEVICE_ATTR the MTCR is set by using the
+attr->attr value KVM_S390_VM_CPU_TOPO_MTCR_SET and cleared by
+using KVM_S390_VM_CPU_TOPO_MTCR_CLEAR.

I have the feeling that we can drop the two blocks above and we won't loose information.

+/**
+ * kvm_s390_sca_clear_mtcr
+ * @kvm: guest KVM description
+ *
+ * Is only relevant if the topology facility is present,
+ * the caller should check KVM facility 11
+ *
+ * Updates the Multiprocessor Topology-Change-Report to signal
+ * the guest with a topology change.
+ */
+static void kvm_s390_sca_clear_mtcr(struct kvm *kvm)

This is a set operation with the value 0 and that's clearly visible by the copied code. If you make the utility entry a bitfield you can easily set 0/1 via one function without doing the bit manipulation by hand.

OK


I.e. please only use one set function.

+{
+    struct bsca_block *sca = kvm->arch.sca; /* SCA version doesn't matter */
+
+    ipte_lock(kvm);
+    sca->utility &= ~SCA_UTILITY_MTCR;
+    ipte_unlock(kvm);
+}
+
+static int kvm_s390_set_topology(struct kvm *kvm, struct kvm_device_attr *attr)
+{
+    if (!test_kvm_facility(kvm, 11))
+        return -ENXIO;
+
+    switch (attr->attr) {
+    case KVM_S390_VM_CPU_TOPO_MTCR_SET:
+        kvm_s390_sca_set_mtcr(kvm);
+        break;
+    case KVM_S390_VM_CPU_TOPO_MTCR_CLEAR:
+        kvm_s390_sca_clear_mtcr(kvm);
+        break;
+    }

By having two endpoints here we trade an easy check with having to access process memory to grab the value we want to set.

I'm still torn about this.

+    return 0;
+}
+
+/**
+ * kvm_s390_sca_get_mtcr
+ * @kvm: guest KVM description
+ *
+ * Is only relevant if the topology facility is present,
+ * the caller should check KVM facility 11
+ *
+ * reports to QEMU the Multiprocessor Topology-Change-Report.
+ */
+static int kvm_s390_sca_get_mtcr(struct kvm *kvm)
+{
+    struct bsca_block *sca = kvm->arch.sca; /* SCA version doesn't matter */

Same comments as with the set_mtcr()

OK


+    int val;
+
+    ipte_lock(kvm);
+    val = sca->utility & SCA_UTILITY_MTCR;
+    ipte_unlock(kvm);
+
+    return val;
+}
+
+static int kvm_s390_get_topology(struct kvm *kvm, struct kvm_device_attr *attr)
+{
+    struct kvm_cpu_topology topo = {};
+
+    if (!test_kvm_facility(kvm, 11))
+        return -ENXIO;
+
+    topo.mtcr = kvm_s390_sca_get_mtcr(kvm) ? 1 : 0;
+    if (copy_to_user((void __user *)attr->addr, &topo, sizeof(topo)))
+        return -EFAULT;
+
+    return 0;
+}
+
  static int kvm_s390_vm_set_attr(struct kvm *kvm, struct kvm_device_attr *attr)
  {
      int ret;
@@ -1730,6 +1817,9 @@ static int kvm_s390_vm_set_attr(struct kvm *kvm, struct kvm_device_attr *attr)
      case KVM_S390_VM_MIGRATION:
          ret = kvm_s390_vm_set_migration(kvm, attr);
          break;
+    case KVM_S390_VM_CPU_TOPOLOGY:
+        ret = kvm_s390_set_topology(kvm, attr);
+        break;
      default:
          ret = -ENXIO;
          break;
@@ -1755,6 +1845,9 @@ static int kvm_s390_vm_get_attr(struct kvm *kvm, struct kvm_device_attr *attr)
      case KVM_S390_VM_MIGRATION:
          ret = kvm_s390_vm_get_migration(kvm, attr);
          break;
+    case KVM_S390_VM_CPU_TOPOLOGY:
+        ret = kvm_s390_get_topology(kvm, attr);
+        break;
      default:
          ret = -ENXIO;
          break;
@@ -1828,6 +1921,9 @@ static int kvm_s390_vm_has_attr(struct kvm *kvm, struct kvm_device_attr *attr)
      case KVM_S390_VM_MIGRATION:
          ret = 0;
          break;
+    case KVM_S390_VM_CPU_TOPOLOGY:
+        ret = test_kvm_facility(kvm, 11) ? 0 : -ENXIO;
+        break;
      default:
          ret = -ENXIO;
          break;
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 5088bd9f1922..33317d820032 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1157,6 +1157,7 @@ struct kvm_ppc_resize_hpt {
  #define KVM_CAP_VM_TSC_CONTROL 214
  #define KVM_CAP_SYSTEM_EVENT_DATA 215
  #define KVM_CAP_ARM_SYSTEM_SUSPEND 216
+#define KVM_CAP_S390_CPU_TOPOLOGY 217
  #ifdef KVM_CAP_IRQ_ROUTING


--
Pierre Morel
IBM Lab Boeblingen



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