[PATCH v2 1/2] KVM: arm64: Turn kvm_arm_support_pmu_v3() into a static key

[Alexandru Elisei]

Hi Marc,

On 2/9/21 11:48 AM, Marc Zyngier wrote:
> 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.

The patch looks correct to me. I compiled the kernel with CONFIG_ARM_PMU set and
unset, saw no problems. The IS_ENABLED(CONFIG_ARM_PMU) condition does prevent the
compile error reported by Andre, as removing it triggered an error. I also double
checked that static keys are initialized before KVM, which in hindsight was
obvious, since the GIC is initialized before KVM, and the GIC driver makes use of
static keys:

Reviewed-by: Alexandru Elisei <alexandru.elisei at arm.com>

Thanks,

Alex

>
> 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..739164324afe 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 (IS_ENABLED(CONFIG_ARM_PMU) && 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)
>  {