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

Janosch Frank
2022-06-24T06:50:59Z

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@linux.ibm.com>
> ---
> 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.

> +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

> +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.

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()

> + 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
>