Re: [PATCH v3 2/3] s390x: KVM: Implementation of Multiprocessor Topology-Change-Report

David Hildenbrand
2021-09-06T18:37:41Z

On 03.08.21 10:26, Pierre Morel wrote:
> We let the userland hypervisor know if the machine support the CPU
> topology facility using a new KVM capability: KVM_CAP_S390_CPU_TOPOLOGY.
>
> The PTF instruction will report a topology change if there is any change
> with a previous STSI_15_2 SYSIB.
> Changes inside a STSI_15_2 SYSIB occur if CPU bits are set or clear
> inside the CPU Topology List Entry CPU mask field, which happens with
> changes in CPU polarization, dedication, CPU types and adding or
> removing CPUs in a socket.
>
> The reporting to the guest is done using the Multiprocessor
> Topology-Change-Report (MTCR) bit of the utility entry of the guest's
> SCA which will be cleared during the interpretation of PTF.
>
> To check if the topology has been modified we use a new field of the
> arch vCPU to save the previous real CPU ID at the end of a schedule
> and verify on next schedule that the CPU used is in the same socket.
>
> We deliberatly ignore:
> - polarization: only horizontal polarization is currently used in linux.
> - CPU Type: only IFL Type are supported in Linux
> - Dedication: we consider that only a complete dedicated CPU stack can
> take benefit of the CPU Topology.
>
> Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>

> @@ -228,7 +232,7 @@ struct kvm_s390_sie_block {
> __u8 icptcode; /* 0x0050 */
> __u8 icptstatus; /* 0x0051 */
> __u16 ihcpu; /* 0x0052 */
> - __u8 reserved54; /* 0x0054 */
> + __u8 mtcr; /* 0x0054 */
> #define IICTL_CODE_NONE 0x00
> #define IICTL_CODE_MCHK 0x01
> #define IICTL_CODE_EXT 0x02
> @@ -246,6 +250,7 @@ struct kvm_s390_sie_block {
> #define ECB_TE 0x10
> #define ECB_SRSI 0x04
> #define ECB_HOSTPROTINT 0x02
> +#define ECB_PTF 0x01

From below I understand, that ECB_PTF can be used with stfl(11) in the
hypervisor.

What is to happen if the hypervisor doesn't support stfl(11) and we
consequently cannot use ECB_PTF? Will QEMU be able to emulate PTF fully?

> __u8 ecb; /* 0x0061 */
> #define ECB2_CMMA 0x80
> #define ECB2_IEP 0x20
> @@ -747,6 +752,7 @@ struct kvm_vcpu_arch {
> bool skey_enabled;
> struct kvm_s390_pv_vcpu pv;
> union diag318_info diag318_info;
> + int prev_cpu;
> };
>
> struct kvm_vm_stat {
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index b655a7d82bf0..ff6d8a2b511c 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -568,6 +568,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> case KVM_CAP_S390_VCPU_RESETS:
> case KVM_CAP_SET_GUEST_DEBUG:
> case KVM_CAP_S390_DIAG318:
> + case KVM_CAP_S390_CPU_TOPOLOGY:

I would have expected instead

r = test_facility(11);
break

...

> r = 1;
> break;
> case KVM_CAP_SET_GUEST_DEBUG2:
> @@ -819,6 +820,23 @@ 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:
> + mutex_lock(&kvm->lock);
> + if (kvm->created_vcpus) {
> + r = -EBUSY;
> + } else {

...
} 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;
} else {
r = -EINVAL;
}

similar to how we handle KVM_CAP_S390_VECTOR_REGISTERS.

But I assume you want to be able to support hosts without ECB_PTF, correct?

> + 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: CPU TOPOLOGY %s",
> + r ? "(not available)" : "(success)");
> + break;
> +
> + r = -EINVAL;
> + break;

^ dead code

[...]

> }
>
> void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> {
> + vcpu->arch.prev_cpu = vcpu->cpu;
> vcpu->cpu = -1;
> if (vcpu->arch.cputm_enabled && !is_vcpu_idle(vcpu))
> __stop_cpu_timer_accounting(vcpu);
> @@ -3198,6 +3239,11 @@ static int kvm_s390_vcpu_setup(struct kvm_vcpu *vcpu)
> vcpu->arch.sie_block->ecb |= ECB_HOSTPROTINT;
> if (test_kvm_facility(vcpu->kvm, 9))
> vcpu->arch.sie_block->ecb |= ECB_SRSI;
> +
> + /* PTF needs both host and guest facilities to enable interpretation */
> + if (test_kvm_facility(vcpu->kvm, 11) && test_facility(11))
> + vcpu->arch.sie_block->ecb |= ECB_PTF;

Here you say we need both ...

> +
> if (test_kvm_facility(vcpu->kvm, 73))
> vcpu->arch.sie_block->ecb |= ECB_TE;
>
> diff --git a/arch/s390/kvm/vsie.c b/arch/s390/kvm/vsie.c
> index 4002a24bc43a..50d67190bf65 100644
> --- a/arch/s390/kvm/vsie.c
> +++ b/arch/s390/kvm/vsie.c
> @@ -503,6 +503,9 @@ static int shadow_scb(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
> /* Host-protection-interruption introduced with ESOP */
> if (test_kvm_cpu_feat(vcpu->kvm, KVM_S390_VM_CPU_FEAT_ESOP))
> scb_s->ecb |= scb_o->ecb & ECB_HOSTPROTINT;
> + /* CPU Topology */
> + if (test_kvm_facility(vcpu->kvm, 11))
> + scb_s->ecb |= scb_o->ecb & ECB_PTF;

but here you don't check?

> /* transactional execution */
> if (test_kvm_facility(vcpu->kvm, 73) && wants_tx) {
> /* remap the prefix is tx is toggled on */
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index d9e4aabcb31a..081ce0cd44b9 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1112,6 +1112,7 @@ struct kvm_ppc_resize_hpt {
> #define KVM_CAP_BINARY_STATS_FD 203
> #define KVM_CAP_EXIT_ON_EMULATION_FAILURE 204
> #define KVM_CAP_ARM_MTE 205
> +#define KVM_CAP_S390_CPU_TOPOLOGY 206
>

We'll need a Documentation/virt/kvm/api.rst description.

I'm not completely confident that the way we're handling the
capability+facility is the right approach. It all feels a bit suboptimal.

Except stfl(74) -- STHYI --, we never enable a facility via
set_kvm_facility() that's not available in the host. And STHYI is
special such that it is never implemented in hardware.

I'll think about what might be cleaner once I get some more details
about the interaction with stfl(11) in the hypervisor.

--
Thanks,

David / dhildenb