Re: [PATCH v12 3/3] KVM: s390: resetting the Topology-Change-Report

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On 7/11/22 15:22, Janis Schoetterl-Glausch wrote:
On 7/11/22 10:41, 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>

Reviewed-by: Janis Schoetterl-Glausch <scgl@xxxxxxxxxxxxx>


Thanks!

See nits/comments below.

---
  Documentation/virt/kvm/api.rst   | 25 ++++++++++++++
  arch/s390/include/uapi/asm/kvm.h |  1 +
  arch/s390/kvm/kvm-s390.c         | 56 ++++++++++++++++++++++++++++++++
  include/uapi/linux/kvm.h         |  1 +
  4 files changed, 83 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 11e00a46c610..5e086125d8ad 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -7956,6 +7956,31 @@ 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
+PTF instruction with function codes 0 or 1 and the STSI(15,1,x)

Is the architecture allowed to extend STSI without a facility?
If so, if we say here that STSI 15.1.x is passed to user space, then
I think we should have a

if (sel1 != 1)
	goto out_no_data;

or maybe even

if (sel1 != 1 || sel2 < 2 || sel2 > 6)
	goto out_no_data;

in priv.c

I am not a big fan of doing everything in the kernel.
Here we have no performance issue since it is an error of the guest if it sends a wrong selector.

Even testing the facility or PV in the kernel is for my opinion arguable in the case we do not do any treatment in the kernel.

I do not see what it brings to us, it increase the LOCs and makes the implementation less easy to evolve.



+instruction to the userland hypervisor.
+
+The stfle facility 11, CPU Topology facility, should not be indicated
+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

get or set, now that there is no explicit clear anymore.

Yes now it is a set to 0 but the action of clearing remains.


+Topology Report (MTCR) bit of the SCA through the kvm_device_attr
+structure.> +
+When getting the Modified Change Topology Report value, the attr->addr

When getting/setting the...

+must point to a byte where the value will be stored.

... will be stored/retrieved from.

OK


+
  9. Known KVM API problems
  =========================
diff --git a/arch/s390/include/uapi/asm/kvm.h b/arch/s390/include/uapi/asm/kvm.h
index 7a6b14874d65..a73cf01a1606 100644
--- a/arch/s390/include/uapi/asm/kvm.h
+++ b/arch/s390/include/uapi/asm/kvm.h
@@ -74,6 +74,7 @@ struct kvm_s390_io_adapter_req {
  #define KVM_S390_VM_CRYPTO		2
  #define KVM_S390_VM_CPU_MODEL		3
  #define KVM_S390_VM_MIGRATION		4
+#define KVM_S390_VM_CPU_TOPOLOGY	5
/* kvm attributes for mem_ctrl */
  #define KVM_S390_VM_MEM_ENABLE_CMMA	0
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 70436bfff53a..b18e0b940b26 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -606,6 +606,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
  	case KVM_CAP_S390_PROTECTED:
  		r = is_prot_virt_host();
  		break;
+	case KVM_CAP_S390_CPU_TOPOLOGY:
+		r = test_facility(11);
+		break;
  	default:
  		r = 0;
  	}
@@ -817,6 +820,20 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm, struct kvm_enable_cap *cap)
  		icpt_operexc_on_all_vcpus(kvm);
  		r = 0;
  		break;
+	case KVM_CAP_S390_CPU_TOPOLOGY:
+		r = -EINVAL;
+		mutex_lock(&kvm->lock);
+		if (kvm->created_vcpus) {
+			r = -EBUSY;
+		} else if (test_facility(11)) {
+			set_kvm_facility(kvm->arch.model.fac_mask, 11);
+			set_kvm_facility(kvm->arch.model.fac_list, 11);
+			r = 0;
+		}
+		mutex_unlock(&kvm->lock);
+		VM_EVENT(kvm, 3, "ENABLE: CAP_S390_CPU_TOPOLOGY %s",
+			 r ? "(not available)" : "(success)");
+		break;
  	default:
  		r = -EINVAL;
  		break;
@@ -1717,6 +1734,36 @@ static void kvm_s390_update_topology_change_report(struct kvm *kvm, bool val)
  	read_unlock(&kvm->arch.sca_lock);
  }
+static int kvm_s390_set_topology(struct kvm *kvm, struct kvm_device_attr *attr)

kvm_s390_set_topology_changed maybe?
kvm_s390_get_topology_changed below then.

No strong opinion, if you prefer I change this.


+{
+	if (!test_kvm_facility(kvm, 11))
+		return -ENXIO;
+
+	kvm_s390_update_topology_change_report(kvm, !!attr->attr);
+	return 0;
+}
+
+static int kvm_s390_get_topology(struct kvm *kvm, struct kvm_device_attr *attr)
+{
+	union sca_utility utility;
+	struct bsca_block *sca;
+	__u8 topo;
+
+	if (!test_kvm_facility(kvm, 11))
+		return -ENXIO;
+
+	read_lock(&kvm->arch.sca_lock);
+	sca = kvm->arch.sca;
+	utility.val = READ_ONCE(sca->utility.val);

I don't think you need the READ_ONCE anymore, now that there is a lock it should act as a compile barrier.

I think you are right.

+	read_unlock(&kvm->arch.sca_lock);
+	topo = utility.mtcr;
+
+	if (copy_to_user((void __user *)attr->addr, &topo, sizeof(topo)))

Why void not u8?

I like to say we write on "topo" with the size of "topo".
So we do not need to verify the effective size of topo.
But I understand, it is a UAPI, setting u8 in the copy_to_user makes sense too. For my personal opinion, I would have prefer that userland tell us the size it awaits even here, for this special case, since we use a byte, we can not do really wrong.


+		return -EFAULT;
+
+	return 0;
+}
+
[...]


--
Pierre Morel
IBM Lab Boeblingen



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