Re: [PATCH v8 4/7] target/i386/kvm: query kvm.enable_pmu parameter

["Chen, Zide" <zide.chen@intel.com>]

On 12/29/2025 11:42 PM, Dongli Zhang wrote:
> When PMU is enabled in QEMU, there is a chance that PMU virtualization is
> completely disabled by the KVM module parameter kvm.enable_pmu=N.
> 
> The kvm.enable_pmu parameter is introduced since Linux v5.17.
> Its permission is 0444. It does not change until a reload of the KVM
> module.
> 
> Read the kvm.enable_pmu value from the module sysfs to give a chance to
> provide more information about vPMU enablement.> Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
> Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
> Reviewed-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
> ---
> Changed since v2:
>   - Rework the code flow following Zhao's suggestion.
>   - Return error when:
>     (*kvm_enable_pmu == 'N' && X86_CPU(cpu)->enable_pmu)
> Changed since v3:
>   - Re-split the cases into enable_pmu and !enable_pmu, following Zhao's
>     suggestion.
>   - Rework the commit messages.
>   - Bring back global static variable 'kvm_pmu_disabled' from v2.
> Changed since v4:
>   - Add Reviewed-by from Zhao.
> Changed since v5:
>   - Rebase on top of most recent QEMU.
> Changed since v6:
>   - Add Reviewed-by from Dapeng Mi.
> 
>  target/i386/kvm/kvm.c | 61 +++++++++++++++++++++++++++++++------------
>  1 file changed, 44 insertions(+), 17 deletions(-)
> 
> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
> index 3b803c662d..338b9558e4 100644
> --- a/target/i386/kvm/kvm.c
> +++ b/target/i386/kvm/kvm.c
> @@ -187,6 +187,10 @@ static int has_triple_fault_event;
>  static bool has_msr_mcg_ext_ctl;
>  
>  static int pmu_cap;
> +/*
> + * Read from /sys/module/kvm/parameters/enable_pmu.
> + */
> +static bool kvm_pmu_disabled;
>  
>  static struct kvm_cpuid2 *cpuid_cache;
>  static struct kvm_cpuid2 *hv_cpuid_cache;
> @@ -2068,23 +2072,30 @@ int kvm_arch_pre_create_vcpu(CPUState *cpu, Error **errp)
>      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 (X86_CPU(cpu)->enable_pmu) {
> +            if (kvm_pmu_disabled) {
> +                warn_report("Failed to enable PMU since "
> +                            "KVM's enable_pmu parameter is disabled");

I'm wondering about the intended value of this patch?

If enable_pmu is true in QEMU but the corresponding KVM parameter is
false, then KVM_GET_SUPPORTED_CPUID or KVM_GET_MSRS should be able to
tell that the PMU feature is not supported by host.

The logic implemented in this patch seems somewhat redundant.

Additionally, in this scenario — where the user intends to enable a
feature but the host cannot support it — normally no warning is emitted
by QEMU.

> +            }
> +        } else {
> +            /*
> +             * 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) {
> +                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;
> +                }
>              }
>          }
>      }
> @@ -3302,6 +3313,7 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
>      int ret;
>      struct utsname utsname;
>      Error *local_err = NULL;
> +    g_autofree char *kvm_enable_pmu;
>  
>      /*
>       * Initialize confidential guest (SEV/TDX) context, if required
> @@ -3437,6 +3449,21 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
>  
>      pmu_cap = kvm_check_extension(s, KVM_CAP_PMU_CAPABILITY);
>  
> +    /*
> +     * The enable_pmu parameter is introduced since Linux v5.17,
> +     * give a chance to provide more information about vPMU
> +     * enablement.
> +     *
> +     * The kvm.enable_pmu's permission is 0444. It does not change
> +     * until a reload of the KVM module.
> +     */
> +    if (g_file_get_contents("/sys/module/kvm/parameters/enable_pmu",
> +                            &kvm_enable_pmu, NULL, NULL)) {
> +        if (*kvm_enable_pmu == 'N') {
> +            kvm_pmu_disabled = true;

It’s generally better not to rely on KVM’s internal implementation
unless really necessary.

For example, in the new mediated vPMU framework, even if the KVM module
parameter enable_pmu is set, the per-guest kvm->arch.enable_pmu could
still be cleared.

In such a case, the logic here might not be correct.

> +        }
> +    }
> +
>      return 0;
>  }
>