[PATCH 1/2] KVM: arm64: Turn kvm_arm_support_pmu_v3() into a static key

[Andre Przywara]

On Tue, 26 Jan 2021 15:15:20 +0000
Marc Zyngier <maz at kernel.org> wrote:

Hi Marc,

> We currently find out about the presence of a HW PMU (or the handling
> of that PMU by perf, which amounts to the same thing) in a fairly
> roundabout way, by checking the number of counters available to perf.
> That's good enough for now, but we will soon need to find about about
> that on paths where perf is out of reach (in the world switch).
> 
> Instead, let's turn kvm_arm_support_pmu_v3() into a static key.

I am sure the pesky build bot has told you about it already, but this
fails when ARM_PMU is not defined, as perf_num_counters() is not
defined. It's  bit nasty, since it's a generic function, so we
can't easily stub it in its original header.

Shall we find a place somewhere in arch/arm64 and provide a stub
implementation there, #ifndef CONFIG_ARM_PMU? Sounds ugly, though.

Or something else entirely?

Cheers,
Andre

> Signed-off-by: Marc Zyngier <maz at kernel.org>
> ---
>  arch/arm64/kvm/perf.c     | 10 ++++++++++
>  arch/arm64/kvm/pmu-emul.c | 10 ----------
>  include/kvm/arm_pmu.h     |  9 +++++++--
>  3 files changed, 17 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/arm64/kvm/perf.c b/arch/arm64/kvm/perf.c
> index d45b8b9a4415..198fa4266b2d 100644
> --- a/arch/arm64/kvm/perf.c
> +++ b/arch/arm64/kvm/perf.c
> @@ -11,6 +11,8 @@
>  
>  #include <asm/kvm_emulate.h>
>  
> +DEFINE_STATIC_KEY_FALSE(kvm_arm_pmu_available);
> +
>  static int kvm_is_in_guest(void)
>  {
>          return kvm_get_running_vcpu() != NULL;
> @@ -48,6 +50,14 @@ static struct perf_guest_info_callbacks kvm_guest_cbs = {
>  
>  int kvm_perf_init(void)
>  {
> +	/*
> +	 * Check if HW_PERF_EVENTS are supported by checking the number of
> +	 * hardware performance counters. This could ensure the presence of
> +	 * a physical PMU and CONFIG_PERF_EVENT is selected.
> +	 */
> +	if (perf_num_counters() > 0)
> +		static_branch_enable(&kvm_arm_pmu_available);
> +
>  	return perf_register_guest_info_callbacks(&kvm_guest_cbs);
>  }
>  
> diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> index 4ad66a532e38..44d500706ab9 100644
> --- a/arch/arm64/kvm/pmu-emul.c
> +++ b/arch/arm64/kvm/pmu-emul.c
> @@ -813,16 +813,6 @@ u64 kvm_pmu_get_pmceid(struct kvm_vcpu *vcpu, bool pmceid1)
>  	return val & mask;
>  }
>  
> -bool kvm_arm_support_pmu_v3(void)
> -{
> -	/*
> -	 * Check if HW_PERF_EVENTS are supported by checking the number of
> -	 * hardware performance counters. This could ensure the presence of
> -	 * a physical PMU and CONFIG_PERF_EVENT is selected.
> -	 */
> -	return (perf_num_counters() > 0);
> -}
> -
>  int kvm_arm_pmu_v3_enable(struct kvm_vcpu *vcpu)
>  {
>  	if (!kvm_vcpu_has_pmu(vcpu))
> diff --git a/include/kvm/arm_pmu.h b/include/kvm/arm_pmu.h
> index 8dcb3e1477bc..6fd3cda608e4 100644
> --- a/include/kvm/arm_pmu.h
> +++ b/include/kvm/arm_pmu.h
> @@ -13,6 +13,13 @@
>  #define ARMV8_PMU_CYCLE_IDX		(ARMV8_PMU_MAX_COUNTERS - 1)
>  #define ARMV8_PMU_MAX_COUNTER_PAIRS	((ARMV8_PMU_MAX_COUNTERS + 1) >> 1)
>  
> +DECLARE_STATIC_KEY_FALSE(kvm_arm_pmu_available);
> +
> +static __always_inline bool kvm_arm_support_pmu_v3(void)
> +{
> +	return static_branch_likely(&kvm_arm_pmu_available);
> +}
> +
>  #ifdef CONFIG_HW_PERF_EVENTS
>  
>  struct kvm_pmc {
> @@ -47,7 +54,6 @@ void kvm_pmu_software_increment(struct kvm_vcpu *vcpu, u64 val);
>  void kvm_pmu_handle_pmcr(struct kvm_vcpu *vcpu, u64 val);
>  void kvm_pmu_set_counter_event_type(struct kvm_vcpu *vcpu, u64 data,
>  				    u64 select_idx);
> -bool kvm_arm_support_pmu_v3(void);
>  int kvm_arm_pmu_v3_set_attr(struct kvm_vcpu *vcpu,
>  			    struct kvm_device_attr *attr);
>  int kvm_arm_pmu_v3_get_attr(struct kvm_vcpu *vcpu,
> @@ -87,7 +93,6 @@ static inline void kvm_pmu_software_increment(struct kvm_vcpu *vcpu, u64 val) {}
>  static inline void kvm_pmu_handle_pmcr(struct kvm_vcpu *vcpu, u64 val) {}
>  static inline void kvm_pmu_set_counter_event_type(struct kvm_vcpu *vcpu,
>  						  u64 data, u64 select_idx) {}
> -static inline bool kvm_arm_support_pmu_v3(void) { return false; }
>  static inline int kvm_arm_pmu_v3_set_attr(struct kvm_vcpu *vcpu,
>  					  struct kvm_device_attr *attr)
>  {