Re: [PATCH v5 1/1] s390x: KVM: accept STSI for CPU topology information

Janosch Frank
2021-12-09T16:09:08Z

On 11/22/21 14:14, 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_1_2 SYSIB.
> Changes inside a STSI_15_1_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 assume in this patch:
> - no polarization change: only horizontal polarization is currently
> used in linux.
> - no CPU Type change: only IFL Type are supported in Linux
> - Dedication: with this patch, only a complete dedicated CPU stack can
> take benefit of the CPU Topology.
>
> STSI(15.1.x) gives information on the CPU configuration topology.
> Let's accept the interception of STSI with the function code 15 and
> let the userland part of the hypervisor handle it when userland
> support the CPU Topology facility.
>
> Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
> ---
> Documentation/virt/kvm/api.rst | 16 ++++++++++
> arch/s390/include/asm/kvm_host.h | 14 ++++++---
> arch/s390/kvm/kvm-s390.c | 52 +++++++++++++++++++++++++++++++-
> arch/s390/kvm/priv.c | 7 ++++-
> arch/s390/kvm/vsie.c | 3 ++
> include/uapi/linux/kvm.h | 1 +
> 6 files changed, 87 insertions(+), 6 deletions(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index aeeb071c7688..e5c9da0782a6 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -7484,3 +7484,19 @@ The argument to KVM_ENABLE_CAP is also a bitmask, and must be a subset
> of the result of KVM_CHECK_EXTENSION. KVM will forward to userspace
> the hypercalls whose corresponding bit is in the argument, and return
> ENOSYS for the others.
> +
> +8.17 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) instruction to the userland

The capitalization of "Function code" is inconsistent.

> +hypervisor.
> +
> +The stfle facility 11, CPU Topology facility, should not be provided to the
> +guest without this capability.
> diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
> index a604d51acfc8..cccc09a8fdab 100644
> --- a/arch/s390/include/asm/kvm_host.h
> +++ b/arch/s390/include/asm/kvm_host.h
> @@ -95,15 +95,19 @@ struct bsca_block {
> union ipte_control ipte_control;
> __u64 reserved[5];
> __u64 mcn;
> - __u64 reserved2;
> +#define ESCA_UTILITY_MTCR 0x8000
> + __u16 utility;
> + __u8 reserved2[6];
> struct bsca_entry cpu[KVM_S390_BSCA_CPU_SLOTS];
> };
>
> struct esca_block {
> union ipte_control ipte_control;
> - __u64 reserved1[7];
> + __u64 reserved1[6];
> + __u16 utility;
> + __u8 reserved2[6];
> __u64 mcn[4];
> - __u64 reserved2[20];
> + __u64 reserved3[20];

Note to self: Prime example for a move to reserved member names based on
offsets.

> struct esca_entry cpu[KVM_S390_ESCA_CPU_SLOTS];
> };
>
> @@ -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
> @@ -247,6 +251,7 @@ struct kvm_s390_sie_block {
> #define ECB_SPECI 0x08
> #define ECB_SRSI 0x04
> #define ECB_HOSTPROTINT 0x02
> +#define ECB_PTF 0x01
> __u8 ecb; /* 0x0061 */
> #define ECB2_CMMA 0x80
> #define ECB2_IEP 0x20
> @@ -748,6 +753,7 @@ 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 {

[..]

> }
>
> -void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> +static void kvm_s390_set_mtcr(struct kvm_vcpu *vcpu)

We change a vcpu related data structure, there should be "vcpu" in the
function name to indicate that.

> {
> + struct esca_block *esca = vcpu->kvm->arch.sca;
>
> + if (vcpu->arch.sie_block->ecb & ECB_PTF) {

I'm wondering if we should replace these checks with the
test_kvm_facility() ones. ECB_PTF is never changed after vcpu setup, right?

> + ipte_lock(vcpu);
> + WRITE_ONCE(esca->utility, ESCA_UTILITY_MTCR);
> + ipte_unlock(vcpu);
> + }
> +}
> +
> +void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> +{
> gmap_enable(vcpu->arch.enabled_gmap);
> kvm_s390_set_cpuflags(vcpu, CPUSTAT_RUNNING);
> if (vcpu->arch.cputm_enabled && !is_vcpu_idle(vcpu))
> __start_cpu_timer_accounting(vcpu);
> vcpu->cpu = cpu;
> +
> + /*
> + * With PTF interpretation the guest will be aware of topology
> + * change when the Multiprocessor Topology-Change-Report is pending.
> + * We check for events modifying the result of STSI_15_2:
> + * - A new vCPU has been hotplugged (prev_cpu == -1)
> + * - The real CPU backing up the vCPU moved to another socket
> + */
> + if (vcpu->arch.sie_block->ecb & ECB_PTF) {
> + if (vcpu->arch.prev_cpu == -1 ||
> + (topology_physical_package_id(cpu) !=
> + topology_physical_package_id(vcpu->arch.prev_cpu)))

This is barely readable, might be good to put this check in a separate
function in kvm-s390.h.

> + kvm_s390_set_mtcr(vcpu);
> + }
> }
>
> void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> {
> + /* Remember which CPU was backing the vCPU */
> + vcpu->arch.prev_cpu = vcpu->cpu;
> vcpu->cpu = -1;
> if (vcpu->arch.cputm_enabled && !is_vcpu_idle(vcpu))
> __stop_cpu_timer_accounting(vcpu);
> @@ -3220,6 +3263,13 @@ 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 guest facilities to enable interpretation */

Please explain.
How is this different from any other facility a few lines above in this
function?

> + if (test_kvm_facility(vcpu->kvm, 11))
> + vcpu->arch.sie_block->ecb |= ECB_PTF;
> + /* Set the prev_cpu value to an impossible value to detect a new vcpu */

We can either change this to:
"A prev_value of -1 indicates this is a new vcpu"

Or we define a constant which will also make the check in
kvm_arch_vcpu_load() easier to understand.

> + vcpu->arch.prev_cpu = -1;
> +
> if (test_kvm_facility(vcpu->kvm, 73))
> vcpu->arch.sie_block->ecb |= ECB_TE;
> if (!kvm_is_ucontrol(vcpu->kvm))
> diff --git a/arch/s390/kvm/priv.c b/arch/s390/kvm/priv.c
> index 417154b314a6..26d165733496 100644
> --- a/arch/s390/kvm/priv.c
> +++ b/arch/s390/kvm/priv.c
> @@ -861,7 +861,8 @@ static int handle_stsi(struct kvm_vcpu *vcpu)
> if (vcpu->arch.sie_block->gpsw.mask & PSW_MASK_PSTATE)
> return kvm_s390_inject_program_int(vcpu, PGM_PRIVILEGED_OP);
>
> - if (fc > 3) {
> + if ((fc > 3 && fc != 15) ||
> + (fc == 15 && !test_kvm_facility(vcpu->kvm, 11))) {
> kvm_s390_set_psw_cc(vcpu, 3);
> return 0;
> }

How about:

if (fc > 3 && fc != 15)
goto out_no_data;

/* fc 15 is provided with PTF/CPU topology support */
if (fc == 15 && !test_kvm_facility(vcpu->kvm, 11))
goto out_no_data;

> @@ -898,6 +899,10 @@ static int handle_stsi(struct kvm_vcpu *vcpu)
> goto out_no_data;
> handle_stsi_3_2_2(vcpu, (void *) mem);
> break;
> + case 15:
> + trace_kvm_s390_handle_stsi(vcpu, fc, sel1, sel2, operand2);
> + insert_stsi_usr_data(vcpu, operand2, ar, fc, sel1, sel2);
> + return -EREMOTE;
> }
> if (kvm_s390_pv_cpu_is_protected(vcpu)) {
> memcpy((void *)sida_origin(vcpu->arch.sie_block), (void *)mem,
> diff --git a/arch/s390/kvm/vsie.c b/arch/s390/kvm/vsie.c
> index acda4b6fc851..da0397cf2cc7 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;
> /* 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 1daa45268de2..273c62dfbe9a 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1131,6 +1131,7 @@ struct kvm_ppc_resize_hpt {
> #define KVM_CAP_EXIT_ON_EMULATION_FAILURE 204
> #define KVM_CAP_ARM_MTE 205
> #define KVM_CAP_VM_MOVE_ENC_CONTEXT_FROM 206
> +#define KVM_CAP_S390_CPU_TOPOLOGY 207
>
> #ifdef KVM_CAP_IRQ_ROUTING
>
>