Re: [PATCH v6 3/9] target/i386/kvm: set KVM_PMU_CAP_DISABLE if "-pmu" is configured

["Mi, Dapeng" <dapeng1.mi@linux.intel.com>]

On 6/24/2025 3:43 PM, Dongli Zhang wrote:
> Although AMD PERFCORE and PerfMonV2 are removed when "-pmu" is configured,
> there is no way to fully disable KVM AMD PMU virtualization. Neither
> "-cpu host,-pmu" nor "-cpu EPYC" achieves this.
>
> As a result, the following message still appears in the VM dmesg:
>
> [    0.263615] Performance Events: AMD PMU driver.
>
> However, the expected output should be:
>
> [    0.596381] Performance Events: PMU not available due to virtualization, using software events only.
> [    0.600972] NMI watchdog: Perf NMI watchdog permanently disabled
>
> This occurs because AMD does not use any CPUID bit to indicate PMU
> availability.
>
> To address this, KVM_CAP_PMU_CAPABILITY is used to set KVM_PMU_CAP_DISABLE
> when "-pmu" is configured.
>
> Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
> Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
> Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
> ---
> Changed since v1:
>   - Switch back to the initial implementation with "-pmu".
> https://lore.kernel.org/all/20221119122901.2469-3-dongli.zhang@oracle.com
>   - Mention that "KVM_PMU_CAP_DISABLE doesn't change the PMU behavior on
>     Intel platform because current "pmu" property works as expected."
> Changed since v2:
>   - Change has_pmu_cap to pmu_cap.
>   - Use (pmu_cap & KVM_PMU_CAP_DISABLE) instead of only pmu_cap in if
>     statement.
>   - Add Reviewed-by from Xiaoyao and Zhao as the change is minor.
> Changed since v5:
>   - Re-base on top of most recent mainline QEMU.
>   - To resolve conflicts, move the PMU related code before the
>     call site of is_tdx_vm().
>
>  target/i386/kvm/kvm.c | 31 +++++++++++++++++++++++++++++++
>  1 file changed, 31 insertions(+)
>
> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
> index 234878c613..15155b79b5 100644
> --- a/target/i386/kvm/kvm.c
> +++ b/target/i386/kvm/kvm.c
> @@ -178,6 +178,8 @@ static int has_triple_fault_event;
>  
>  static bool has_msr_mcg_ext_ctl;
>  
> +static int pmu_cap;
> +
>  static struct kvm_cpuid2 *cpuid_cache;
>  static struct kvm_cpuid2 *hv_cpuid_cache;
>  static struct kvm_msr_list *kvm_feature_msrs;
> @@ -2062,6 +2064,33 @@ full:
>  
>  int kvm_arch_pre_create_vcpu(CPUState *cpu, Error **errp)
>  {
> +    static bool first = true;
> +    int ret;
> +
> +    if (first) {
> +        first = false;
> +
> +        /*
> +         * Since Linux v5.18, KVM provides a VM-level capability to easily
> +         * disable PMUs; however, QEMU has been providing PMU property per
> +         * CPU since v1.6. In order to accommodate both, have to configure
> +         * the VM-level capability here.
> +         *
> +         * KVM_PMU_CAP_DISABLE doesn't change the PMU
> +         * behavior on Intel platform because current "pmu" property works
> +         * as expected.
> +         */
> +        if ((pmu_cap & KVM_PMU_CAP_DISABLE) && !X86_CPU(cpu)->enable_pmu) {
> +            ret = kvm_vm_enable_cap(kvm_state, KVM_CAP_PMU_CAPABILITY, 0,
> +                                    KVM_PMU_CAP_DISABLE);
> +            if (ret < 0) {
> +                error_setg_errno(errp, -ret,
> +                                 "Failed to set KVM_PMU_CAP_DISABLE");
> +                return ret;
> +            }
> +        }
> +    }
> +
>      if (is_tdx_vm()) {
>          return tdx_pre_create_vcpu(cpu, errp);
>      }
> @@ -3363,6 +3392,8 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
>          }
>      }
>  
> +    pmu_cap = kvm_check_extension(s, KVM_CAP_PMU_CAPABILITY);
> +
>      return 0;
>  }
>  

LGTM.

Reviewed-by: Dapeng Mi <dapeng1.mi@linux.intel.com>