Re: [PATCH 04/15] KVM: x86/pmu: Add PMC bitmap accessor helpers

[Sean Christopherson]

On Tue, Jul 07, 2026, Zide Chen wrote:
> pmu->nr_arch_{gp,fixed}_counters is not able to represent that a PMU
> may include non-contiguous GP or fixed counters.
> 
> pmu->all_valid_pmc_mask already holds a bitmap indicating both fixed
> and general-purpose counters, and loops over valid counters can be
> done via pmu->all_valid_pmc_mask alone. Extend it to a union so that
> the u64 alias is available for convenient mask arithmetic operations.
> 
> Add the necessary helpers to prepare for bitmap-based PMC counter
> implementation.
> 
> No functional change intended.
> 
> Co-developed-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>
> Signed-off-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>
> Signed-off-by: Zide Chen <zide.chen@xxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h |  5 ++-
>  arch/x86/kvm/pmu.h              | 55 +++++++++++++++++++++++++++++----
>  2 files changed, 53 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 395b6f20e9ac..f648dc168685 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -643,7 +643,10 @@ struct kvm_pmu {
>  		DECLARE_BITMAP(reprogram_pmi, X86_PMC_IDX_MAX);
>  		atomic64_t __reprogram_pmi;
>  	};
> -	DECLARE_BITMAP(all_valid_pmc_mask, X86_PMC_IDX_MAX);
> +	union {
> +		DECLARE_BITMAP(all_valid_pmc_mask, X86_PMC_IDX_MAX);
> +		u64 all_valid_pmc_mask64;
> +	};
>  	DECLARE_BITMAP(pmc_in_use, X86_PMC_IDX_MAX);
>  
>  	DECLARE_BITMAP(pmc_counting_instructions, X86_PMC_IDX_MAX);
> diff --git a/arch/x86/kvm/pmu.h b/arch/x86/kvm/pmu.h
> index cdbefda844b9..95dc95a9ae37 100644
> --- a/arch/x86/kvm/pmu.h
> +++ b/arch/x86/kvm/pmu.h
> @@ -88,6 +88,32 @@ static inline bool kvm_vcpu_has_mediated_pmu(struct kvm_vcpu *vcpu)
>  	return enable_mediated_pmu && vcpu_to_pmu(vcpu)->version;
>  }
>  
> +static inline unsigned long kvm_gp_pmc_mask(struct kvm_pmu *pmu)
> +{
> +	return pmu->all_valid_pmc_mask64 &
> +	       GENMASK_ULL(KVM_MAX_NR_GP_COUNTERS - 1, 0);
> +}
> +
> +static inline unsigned long kvm_fixed_pmc_mask(struct kvm_pmu *pmu)
> +{
> +	return (pmu->all_valid_pmc_mask64 >> KVM_FIXED_PMC_BASE_IDX) &
> +		GENMASK_ULL(KVM_MAX_NR_FIXED_COUNTERS - 1, 0);
> +}
> +
> +static inline bool kvm_gp_pmc_supported(struct kvm_pmu *pmu, unsigned int idx)

kvm_is_gp_pmc_supported()
    ^^

> +{
> +	unsigned long bitmap = kvm_gp_pmc_mask(pmu);
> +
> +	return idx < KVM_MAX_NR_GP_COUNTERS && test_bit(idx, &bitmap);

Why generate the full bitmap?  Isn't this equivalent in practice?

	return idx < KVM_MAX_NR_GP_COUNTERS &&
	       test_bit(idx, pmu->all_valid_pmc_mask);

> +static inline bool kvm_fixed_pmc_supported(struct kvm_pmu *pmu, unsigned int idx)
> +{
> +	unsigned long bitmap = kvm_fixed_pmc_mask(pmu);
> +
> +	return idx < KVM_MAX_NR_FIXED_COUNTERS && test_bit(idx, &bitmap);
> +}
> +
>  /*
>   * KVM tracks all counters in 64-bit bitmaps, with general purpose counters
>   * mapped to bits 31:0 and fixed counters mapped to 63:32, e.g. fixed counter 0
> @@ -104,11 +130,11 @@ static inline bool kvm_vcpu_has_mediated_pmu(struct kvm_vcpu *vcpu)
>   */
>  static inline struct kvm_pmc *kvm_pmc_idx_to_pmc(struct kvm_pmu *pmu, int idx)
>  {
> -	if (idx < pmu->nr_arch_gp_counters)
> +	if (kvm_gp_pmc_supported(pmu, idx))
>  		return &pmu->gp_counters[idx];
>  
>  	idx -= KVM_FIXED_PMC_BASE_IDX;
> -	if (idx >= 0 && idx < pmu->nr_arch_fixed_counters)
> +	if (kvm_fixed_pmc_supported(pmu, idx))
>  		return &pmu->fixed_counters[idx];
>  
>  	return NULL;
> @@ -120,6 +146,17 @@ static inline struct kvm_pmc *kvm_pmc_idx_to_pmc(struct kvm_pmu *pmu, int idx)
>  			continue;				\
>  		else						\
>  
> +/*
> + * @mask must be an lvalue of type unsigned long because for_each_set_bit()
> + * takes its address.
> + *
> + * @type is token-pasted into KVM_MAX_NR_##type##_COUNTERS to match one of the
> + * counter defines, e.g. GP, FIXED, AMD_GP, INTEL_GP, or INTEL_FIXED. This
> + * reflects what KVM supports, not the underlying host's PMU capabilities.
> + */
> +#define kvm_for_each_set_pmc_idx(i, mask, type)		\
> +	for_each_set_bit((i), &(mask), KVM_MAX_NR_##type##_COUNTERS)


Provide dedciated macros instead of requiring the caller to pass in a type and
then doing token pasting, e.g.

  #define kvm_for_each_gp_counter()

  #define kvm_for_each_fixed_counter()

>  static inline u64 pmc_bitmask(struct kvm_pmc *pmc)
>  {
>  	struct kvm_pmu *pmu = pmc_to_pmu(pmc);
> @@ -168,9 +205,12 @@ static inline bool kvm_valid_perf_global_ctrl(struct kvm_pmu *pmu,
>  static inline struct kvm_pmc *get_gp_pmc(struct kvm_pmu *pmu, u32 msr,
>  					 u32 base)
>  {
> -	if (msr >= base && msr < base + pmu->nr_arch_gp_counters) {
> +	if (msr >= base && msr < base + KVM_MAX_NR_GP_COUNTERS) {
>  		u32 index = array_index_nospec(msr - base,
> -					       pmu->nr_arch_gp_counters);
> +					       KVM_MAX_NR_GP_COUNTERS);
> +
> +		if (!kvm_gp_pmc_supported(pmu, index))
> +			return NULL;
>  
>  		return &pmu->gp_counters[index];
>  	}
> @@ -183,9 +223,12 @@ static inline struct kvm_pmc *get_fixed_pmc(struct kvm_pmu *pmu, u32 msr)
>  {
>  	int base = MSR_CORE_PERF_FIXED_CTR0;
>  
> -	if (msr >= base && msr < base + pmu->nr_arch_fixed_counters) {
> +	if (msr >= base && msr < base + KVM_MAX_NR_FIXED_COUNTERS) {
>  		u32 index = array_index_nospec(msr - base,
> -					       pmu->nr_arch_fixed_counters);
> +					       KVM_MAX_NR_FIXED_COUNTERS);
> +
> +		if (!kvm_fixed_pmc_supported(pmu, index))
> +			return NULL;
>  
>  		return &pmu->fixed_counters[index];
>  	}
> -- 
> 2.54.0
>