Re: [PATCH RESEND v2 1/2] target/i386/kvm: introduce 'pmu-cap-disabled' to set KVM_PMU_CAP_DISABLE

[Like Xu <like.xu.linux@gmail.com>]

On Wed, Jun 21, 2023 at 9:39 AM Dongli Zhang <dongli.zhang@oracle.com> wrote:
>
> The "perf stat" at the VM side still works even we set "-cpu host,-pmu" in
> the QEMU command line. That is, neither "-cpu host,-pmu" nor "-cpu EPYC"
> could disable the pmu virtualization in an AMD environment.
>
> We still see below at VM kernel side ...
>
> [    0.510611] Performance Events: Fam17h+ core perfctr, AMD PMU driver.
>
> ... although we expect something like below.
>
> [    0.596381] Performance Events: PMU not available due to virtualization, using software events only.
> [    0.600972] NMI watchdog: Perf NMI watchdog permanently disabled
>
> This is because the AMD pmu (v1) does not rely on cpuid to decide if the
> pmu virtualization is supported.
>
> We introduce a new property 'pmu-cap-disabled' for KVM accel to set
> KVM_PMU_CAP_DISABLE if KVM_CAP_PMU_CAPABILITY is supported. Only x86 host
> is supported because currently KVM uses KVM_CAP_PMU_CAPABILITY only for
> x86.

We may check cpu->enable_pmu when creating the first CPU or a BSP one
(before it gets running) and then choose whether to disable guest pmu using
vm ioctl KVM_CAP_PMU_CAPABILITY. Introducing a new property is not too
acceptable if there are other options.

>
> Cc: Joe Jin <joe.jin@oracle.com>
> Cc: Like Xu <likexu@tencent.com>
> Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
> ---
> Changed since v1:
> - In version 1 we did not introduce the new property. We ioctl
>   KVM_PMU_CAP_DISABLE only before the creation of the 1st vcpu. We had
>   introduced a helpfer function to do this job before creating the 1st
>   KVM vcpu in v1.
>
>  accel/kvm/kvm-all.c      |  1 +
>  include/sysemu/kvm_int.h |  1 +
>  qemu-options.hx          |  7 ++++++
>  target/i386/kvm/kvm.c    | 46 ++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 55 insertions(+)
>
> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> index 7679f397ae..238098e991 100644
> --- a/accel/kvm/kvm-all.c
> +++ b/accel/kvm/kvm-all.c
> @@ -3763,6 +3763,7 @@ static void kvm_accel_instance_init(Object *obj)
>      s->xen_version = 0;
>      s->xen_gnttab_max_frames = 64;
>      s->xen_evtchn_max_pirq = 256;
> +    s->pmu_cap_disabled = false;
>  }
>
>  /**
> diff --git a/include/sysemu/kvm_int.h b/include/sysemu/kvm_int.h
> index 511b42bde5..cbbe08ec54 100644
> --- a/include/sysemu/kvm_int.h
> +++ b/include/sysemu/kvm_int.h
> @@ -123,6 +123,7 @@ struct KVMState
>      uint32_t xen_caps;
>      uint16_t xen_gnttab_max_frames;
>      uint16_t xen_evtchn_max_pirq;
> +    bool pmu_cap_disabled;
>  };
>
>  void kvm_memory_listener_register(KVMState *s, KVMMemoryListener *kml,
> diff --git a/qemu-options.hx b/qemu-options.hx
> index b57489d7ca..1976c0ca3e 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -187,6 +187,7 @@ DEF("accel", HAS_ARG, QEMU_OPTION_accel,
>      "                tb-size=n (TCG translation block cache size)\n"
>      "                dirty-ring-size=n (KVM dirty ring GFN count, default 0)\n"
>      "                notify-vmexit=run|internal-error|disable,notify-window=n (enable notify VM exit and set notify window, x86 only)\n"
> +    "                pmu-cap-disabled=true|false (disable KVM_CAP_PMU_CAPABILITY, x86 only, default false)\n"
>      "                thread=single|multi (enable multi-threaded TCG)\n", QEMU_ARCH_ALL)
>  SRST
>  ``-accel name[,prop=value[,...]]``
> @@ -254,6 +255,12 @@ SRST
>          open up for a specified of time (i.e. notify-window).
>          Default: notify-vmexit=run,notify-window=0.
>
> +    ``pmu-cap-disabled=true|false``
> +        When the KVM accelerator is used, it controls whether to disable the
> +        KVM_CAP_PMU_CAPABILITY via KVM_PMU_CAP_DISABLE. When disabled, the
> +        PMU virtualization is disabled at the KVM module side. This is for
> +        x86 host only.
> +
>  ERST
>
>  DEF("smp", HAS_ARG, QEMU_OPTION_smp,
> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
> index de531842f6..bf4136fa1b 100644
> --- a/target/i386/kvm/kvm.c
> +++ b/target/i386/kvm/kvm.c
> @@ -129,6 +129,7 @@ static bool has_msr_ucode_rev;
>  static bool has_msr_vmx_procbased_ctls2;
>  static bool has_msr_perf_capabs;
>  static bool has_msr_pkrs;
> +static bool has_pmu_cap;
>
>  static uint32_t has_architectural_pmu_version;
>  static uint32_t num_architectural_pmu_gp_counters;
> @@ -2767,6 +2768,23 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
>          }
>      }
>
> +    has_pmu_cap = kvm_check_extension(s, KVM_CAP_PMU_CAPABILITY);
> +
> +    if (s->pmu_cap_disabled) {
> +        if (has_pmu_cap) {
> +            ret = kvm_vm_enable_cap(s, KVM_CAP_PMU_CAPABILITY, 0,
> +                                    KVM_PMU_CAP_DISABLE);
> +            if (ret < 0) {
> +                s->pmu_cap_disabled = false;
> +                error_report("kvm: Failed to disable pmu cap: %s",
> +                             strerror(-ret));
> +            }
> +        } else {
> +            s->pmu_cap_disabled = false;
> +            error_report("kvm: KVM_CAP_PMU_CAPABILITY is not supported");
> +        }
> +    }
> +
>      return 0;
>  }
>
> @@ -5951,6 +5969,28 @@ static void kvm_arch_set_xen_evtchn_max_pirq(Object *obj, Visitor *v,
>      s->xen_evtchn_max_pirq = value;
>  }
>
> +static void kvm_set_pmu_cap_disabled(Object *obj, Visitor *v,
> +                                     const char *name, void *opaque,
> +                                     Error **errp)
> +{
> +    KVMState *s = KVM_STATE(obj);
> +    bool pmu_cap_disabled;
> +    Error *error = NULL;
> +
> +    if (s->fd != -1) {
> +        error_setg(errp, "Cannot set properties after the accelerator has been initialized");
> +        return;
> +    }
> +
> +    visit_type_bool(v, name, &pmu_cap_disabled, &error);
> +    if (error) {
> +        error_propagate(errp, error);
> +        return;
> +    }
> +
> +    s->pmu_cap_disabled = pmu_cap_disabled;
> +}
> +
>  void kvm_arch_accel_class_init(ObjectClass *oc)
>  {
>      object_class_property_add_enum(oc, "notify-vmexit", "NotifyVMexitOption",
> @@ -5990,6 +6030,12 @@ void kvm_arch_accel_class_init(ObjectClass *oc)
>                                NULL, NULL);
>      object_class_property_set_description(oc, "xen-evtchn-max-pirq",
>                                            "Maximum number of Xen PIRQs");
> +
> +    object_class_property_add(oc, "pmu-cap-disabled", "bool",
> +                              NULL, kvm_set_pmu_cap_disabled,
> +                              NULL, NULL);
> +    object_class_property_set_description(oc, "pmu-cap-disabled",
> +                                          "Disable KVM_CAP_PMU_CAPABILITY");
>  }
>
>  void kvm_set_max_apic_id(uint32_t max_apic_id)
> --
> 2.34.1
>