linux-kernel - Re: [PATCH] perf tool: arch: arm64: change the way for get_cpuid

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Date:   Fri, 19 Jul 2019 08:54:50 +0100
From:   Will Deacon <will@...nel.org>
To:     Joakim Zhang <qiangqing.zhang@....com>
Cc:     "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "acme@...nel.org" <acme@...nel.org>,
        "jolsa@...hat.com" <jolsa@...hat.com>,
        "namhyung@...nel.org" <namhyung@...nel.org>,
        Frank Li <frank.li@....com>, dl-linux-imx <linux-imx@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        mark.rutland@....com
Subject: Re: [PATCH] perf tool: arch: arm64: change the way for
 get_cpuid_str() function

On Thu, Jul 18, 2019 at 06:21:30AM +0000, Joakim Zhang wrote:
> Now the get_cpuid__str function returns the MIDR string of the first
> online cpu from the range of cpus asscociated with the PMU CORE device.
> 
> It can work when pass a perf_pmu entity to get_cpuid_str. However, it
> will pass NULL via perf_pmu__find_map from metricgroup.c if we want to add
> metric group for arm64. When pass NULL to get_cpuid_str, it can't return
> the MIDR string.

Why is this code passing a NULL PMU? What information does it actually
need? Other functions, such as print_pmu_events(), iterate over all PMUs.

> There are three methods from userspace getting MIDR string for arm64:
> 1. parse sysfs(/sys/devices/system/cpu/cpu?/regs/identification/midr_el1)
> 2. parse procfs(/proc/cpuinfo)
> 3. read the hwcaps(MIDR register) with getauxval(AT_HWCAP)
> 
> Perfer to select #3 as it is more simple and direct.

That's probably terminally broken for heterogeneous systems, so I'm not
at all happy with this patch.

Will

> diff --git a/tools/perf/arch/arm64/util/header.c b/tools/perf/arch/arm64/util/header.c
> index 534cd2507d83..f58f08af0be8 100644
> --- a/tools/perf/arch/arm64/util/header.c
> +++ b/tools/perf/arch/arm64/util/header.c
> @@ -2,64 +2,46 @@
>  #include <stdlib.h>
>  #include <api/fs/fs.h>
>  #include "header.h"
> +#include <asm/hwcap.h>
> +#include <sys/auxv.h>
>  
> -#define MIDR "/regs/identification/midr_el1"
>  #define MIDR_SIZE 19
>  #define MIDR_REVISION_MASK      0xf
>  #define MIDR_VARIANT_SHIFT      20
>  #define MIDR_VARIANT_MASK       (0xf << MIDR_VARIANT_SHIFT)
>  
> -char *get_cpuid_str(struct perf_pmu *pmu)
> +#define get_cpuid(id) ({					\
> +		unsigned long __val;				\
> +		asm("mrs %0, "#id : "=r" (__val));		\
> +		__val;						\
> +	})
> +
> +char *get_cpuid_str(struct perf_pmu *pmu __maybe_unused)
>  {
>  	char *buf = NULL;
> -	char path[PATH_MAX];
> -	const char *sysfs = sysfs__mountpoint();
> -	int cpu;
> -	u64 midr = 0;
> -	struct cpu_map *cpus;
> -	FILE *file;
> +	unsigned long midr = 0;
>  
> -	if (!sysfs || !pmu || !pmu->cpus)
> +	if (!(getauxval(AT_HWCAP) & HWCAP_CPUID)) {
> +		fputs("CPUID registers unavailable\n", stderr);
>  		return NULL;
> +	}
>  
> -	buf = malloc(MIDR_SIZE);
> -	if (!buf)
> +	midr = get_cpuid(MIDR_EL1);
> +	if (!midr) {
> +		fputs("Failed to get cpuid string\n", stderr);
>  		return NULL;
> +	}
>  
> -	/* read midr from list of cpus mapped to this pmu */
> -	cpus = cpu_map__get(pmu->cpus);
> -	for (cpu = 0; cpu < cpus->nr; cpu++) {
> -		scnprintf(path, PATH_MAX, "%s/devices/system/cpu/cpu%d"MIDR,
> -				sysfs, cpus->map[cpu]);
> -
> -		file = fopen(path, "r");
> -		if (!file) {
> -			pr_debug("fopen failed for file %s\n", path);
> -			continue;
> -		}
> -
> -		if (!fgets(buf, MIDR_SIZE, file)) {
> -			fclose(file);
> -			continue;
> -		}
> -		fclose(file);
> +	/* Ignore/clear Variant[23:20] and
> +	 * Revision[3:0] of MIDR
> +	 */
> +	midr &= (~(MIDR_VARIANT_MASK | MIDR_REVISION_MASK));
>  
> -		/* Ignore/clear Variant[23:20] and
> -		 * Revision[3:0] of MIDR
> -		 */
> -		midr = strtoul(buf, NULL, 16);
> -		midr &= (~(MIDR_VARIANT_MASK | MIDR_REVISION_MASK));
> -		scnprintf(buf, MIDR_SIZE, "0x%016lx", midr);
> -		/* got midr break loop */
> -		break;
> -	}
> +	buf = malloc(MIDR_SIZE);
> +	if (!buf)
> +		return NULL;
>  
> -	if (!midr) {
> -		pr_err("failed to get cpuid string for PMU %s\n", pmu->name);
> -		free(buf);
> -		buf = NULL;
> -	}
> +	scnprintf(buf, MIDR_SIZE, "0x%016lx", midr);
>  
> -	cpu_map__put(cpus);
>  	return buf;
>  }
> -- 
> 2.17.1
>