Re: [PATCH 2/3] i386: kvm: disable KVM_CAP_PMU_CAPABILITY if "pmu" is disabled

[Greg Kurz <groug@kaod.org>]

On Sat, 19 Nov 2022 04:29:00 -0800
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 disable KVM_CAP_PMU_CAPABILITY if the 'pmu' is disabled in the vcpu
> properties.
> 
> Cc: Joe Jin <joe.jin@oracle.com>
> Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
> ---
>  target/i386/kvm/kvm.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
> index 8fec0bc5b5..0b1226ff7f 100644
> --- a/target/i386/kvm/kvm.c
> +++ b/target/i386/kvm/kvm.c
> @@ -137,6 +137,8 @@ static int has_triple_fault_event;
>  
>  static bool has_msr_mcg_ext_ctl;
>  
> +static int has_pmu_cap;
> +
>  static struct kvm_cpuid2 *cpuid_cache;
>  static struct kvm_cpuid2 *hv_cpuid_cache;
>  static struct kvm_msr_list *kvm_feature_msrs;
> @@ -1725,6 +1727,19 @@ static void kvm_init_nested_state(CPUX86State *env)
>  
>  void kvm_arch_pre_create_vcpu(CPUState *cs)
>  {
> +    X86CPU *cpu = X86_CPU(cs);
> +    int ret;
> +
> +    if (has_pmu_cap && !cpu->enable_pmu) {
> +        ret = kvm_vm_enable_cap(kvm_state, KVM_CAP_PMU_CAPABILITY, 0,
> +                                KVM_PMU_CAP_DISABLE);

It doesn't seem conceptually correct to configure VM level stuff out of
a vCPU property, which could theoretically be different for each vCPU,
even if this isn't the case with the current code base.

Maybe consider controlling PMU with a machine property and this
could be done in kvm_arch_init() like other VM level stuff ?

> +        if (ret < 0) {
> +            error_report("kvm: Failed to disable pmu cap: %s",
> +                         strerror(-ret));
> +        }
> +
> +        has_pmu_cap = 0;
> +    }
>  }
>  
>  int kvm_arch_init_vcpu(CPUState *cs)
> @@ -2517,6 +2532,8 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
>          }
>      }
>  
> +    has_pmu_cap = kvm_check_extension(s, KVM_CAP_PMU_CAPABILITY);
> +
>      ret = kvm_get_supported_msrs(s);
>      if (ret < 0) {
>          return ret;