Re: [PATCH] arm64: cpufeature: Display the set of cores with a feature

[Ionela Voinescu]

Hi,

On Thursday 12 Oct 2023 at 20:30:16 (-0500), Jeremy Linton wrote:
> The AMU feature can be enabled on a subset of the cores in a system.
> Because of that, it prints a message for each core as it is detected.
> This becomes tedious when there are hundreds of cores. Instead, for
> CPU features which can be enabled on a subset of the present cores,
> lets wait until update_cpu_capabilities() and print the subset of cores
> the feature was enabled on.
> 
> Signed-off-by: Jeremy Linton <jeremy.linton@xxxxxxx>
> ---
>  arch/arm64/include/asm/cpufeature.h |  2 ++
>  arch/arm64/kernel/cpufeature.c      | 16 +++++++++++++---
>  2 files changed, 15 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 5bba39376055..19b4d001d845 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -23,6 +23,7 @@
>  #include <linux/bug.h>
>  #include <linux/jump_label.h>
>  #include <linux/kernel.h>
> +#include <linux/cpumask.h>
>  
>  /*
>   * CPU feature register tracking
> @@ -380,6 +381,7 @@ struct arm64_cpu_capabilities {
>  	 * method is robust against being called multiple times.
>  	 */
>  	const struct arm64_cpu_capabilities *match_list;
> +	const struct cpumask *cpus;
>  };
>  
>  static inline int cpucap_default_scope(const struct arm64_cpu_capabilities *cap)
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 444a73c2e638..18711e35924c 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1944,8 +1944,6 @@ int get_cpu_with_amu_feat(void)
>  static void cpu_amu_enable(struct arm64_cpu_capabilities const *cap)
>  {
>  	if (has_cpuid_feature(cap, SCOPE_LOCAL_CPU)) {
> -		pr_info("detected CPU%d: Activity Monitors Unit (AMU)\n",
> -			smp_processor_id());
>  		cpumask_set_cpu(smp_processor_id(), &amu_cpus);
>  
>  		/* 0 reference values signal broken/disabled counters */
> @@ -2411,10 +2409,12 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  		 * message to be shown until at least one CPU is detected to
>  		 * support the feature.
>  		 */
> +		.desc = "Activity Monitors Unit (AMU)",
>  		.capability = ARM64_HAS_AMU_EXTN,
>  		.type = ARM64_CPUCAP_WEAK_LOCAL_CPU_FEATURE,
>  		.matches = has_amu,
>  		.cpu_enable = cpu_amu_enable,
> +		.cpus = &amu_cpus,
>  		ARM64_CPUID_FIELDS(ID_AA64PFR0_EL1, AMU, IMP)
>  	},
>  #endif /* CONFIG_ARM64_AMU_EXTN */
> @@ -2981,7 +2981,7 @@ static void update_cpu_capabilities(u16 scope_mask)
>  		    !caps->matches(caps, cpucap_default_scope(caps)))
>  			continue;
>  
> -		if (caps->desc)
> +		if (caps->desc && !caps->cpus)
>  			pr_info("detected: %s\n", caps->desc);
>  
>  		__set_bit(caps->capability, system_cpucaps);
> @@ -3330,6 +3330,7 @@ unsigned long cpu_get_elf_hwcap2(void)
>  
>  static void __init setup_system_capabilities(void)
>  {
> +	int i;
>  	/*
>  	 * We have finalised the system-wide safe feature
>  	 * registers, finalise the capabilities that depend
> @@ -3338,6 +3339,15 @@ static void __init setup_system_capabilities(void)
>  	 */
>  	update_cpu_capabilities(SCOPE_SYSTEM);
>  	enable_cpu_capabilities(SCOPE_ALL & ~SCOPE_BOOT_CPU);
> +
> +	for (i = 0; i < ARM64_NCAPS; i++) {
> +		const struct arm64_cpu_capabilities *caps = cpucap_ptrs[i];
> +
> +		if (caps && caps->cpus && caps->desc &&
> +			cpumask_any(caps->cpus) < nr_cpu_ids)
> +			pr_info("detected: %s on CPU%*pbl\n",
> +				caps->desc, cpumask_pr_args(caps->cpus));
> +	}
>  }
>  
>  void __init setup_cpu_features(void)
> -- 
> 2.41.0
> 

It looks very nice to me! I gave it a run on an RD-N2 FVP so FWIW:

Reviewed-by: Ionela Voinescu <ionela.voinescu@xxxxxxx>
Tested-by: Ionela Voinescu <ionela.voinescu@xxxxxxx>

The same could be used for DBM, but I'm not sure if creating a mask of
supporting CPUs is worth it, for this purpose only.

Hope it helps,
Ionela.