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SubjectRe: [PATCH v5 2/6] perf arm64: Allow version comparisons of CPU IDs
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On 11/08/2023 15:39, James Clark wrote:
> Currently variant and revision fields are masked out of the MIDR so
> it's not possible to compare different versions of the same CPU.
> In a later commit a workaround will be removed just for N2 r0p3, so
> enable comparisons on version.
>
> This has the side effect of changing the MIDR stored in the header of
> the perf.data file to no longer have masked version fields. It also
> affects the lookups in mapfile.csv, but as that currently only has
> zeroed version fields, it has no actual effect. The mapfile.csv
> documentation also states to zero the version fields, so unless this
> isn't done it will continue to have no effect.
>

This looks ok apart from a couple of comments, below.

Thanks,
John

> Signed-off-by: James Clark <james.clark@arm.com>
> ---
> tools/perf/arch/arm64/util/header.c | 64 ++++++++++++++++++++++-------
> 1 file changed, 50 insertions(+), 14 deletions(-)
>
> diff --git a/tools/perf/arch/arm64/util/header.c b/tools/perf/arch/arm64/util/header.c
> index 80b9f6287fe2..8f74e801e1ab 100644
> --- a/tools/perf/arch/arm64/util/header.c
> +++ b/tools/perf/arch/arm64/util/header.c
> @@ -1,3 +1,6 @@
> +#include <linux/kernel.h>
> +#include <linux/bits.h>
> +#include <linux/bitfield.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <perf/cpumap.h>
> @@ -10,14 +13,12 @@
>
> #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)
> +#define MIDR_REVISION_MASK GENMASK(3, 0)
> +#define MIDR_VARIANT_MASK GENMASK(23, 20)
>
> static int _get_cpuid(char *buf, size_t sz, struct perf_cpu_map *cpus)
> {
> const char *sysfs = sysfs__mountpoint();
> - u64 midr = 0;
> int cpu;
>
> if (!sysfs || sz < MIDR_SIZE)
> @@ -44,21 +45,11 @@ static int _get_cpuid(char *buf, size_t sz, struct perf_cpu_map *cpus)
> }
> fclose(file);
>
> - /* 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;
> }
>
> perf_cpu_map__put(cpus);
> -
> - if (!midr)
> - return EINVAL;

Is there a reason to drop this check?

As I see, it is still checked in perf_pmu__getcpudid() ->
get_cpuid_str() -> _get_cpuid(), and we don't zero the buf allocated in
_get_cpuid()

> -
> return 0;
> }
>
> @@ -99,3 +90,48 @@ char *get_cpuid_str(struct perf_pmu *pmu)
>
> return buf;
> }
> +
> +/*
> + * Return 0 if idstr is a higher or equal to version of the same part as
> + * mapcpuid.

And what other values may be returned? If just 0/1, then can we have a
bool return value?

> + *
> + * Therefore, if mapcpuid has 0 for revision and variant then any version of
> + * idstr will match as long as it's the same CPU type.
> + */
> +int strcmp_cpuid_str(const char *mapcpuid, const char *idstr)
> +{
> + u64 map_id = strtoull(mapcpuid, NULL, 16);
> + char map_id_variant = FIELD_GET(MIDR_VARIANT_MASK, map_id);
> + char map_id_revision = FIELD_GET(MIDR_REVISION_MASK, map_id);
> + u64 id = strtoull(idstr, NULL, 16);
> + char id_variant = FIELD_GET(MIDR_VARIANT_MASK, id);
> + char id_revision = FIELD_GET(MIDR_REVISION_MASK, id);
> + u64 id_fields = ~(MIDR_VARIANT_MASK | MIDR_REVISION_MASK);
> +
> + /* Compare without version first */
> + if ((map_id & id_fields) != (id & id_fields))
> + return 1;
> +
> + /*
> + * ID matches, now compare version.
> + *
> + * Arm revisions (like r0p0) are compared here like two digit semver
> + * values eg. 1.3 < 2.0 < 2.1 < 2.2. The events json file with the
> + * highest matching version is used.
> + *
> + * r = high value = 'Variant' field in MIDR
> + * p = low value = 'Revision' field in MIDR
> + *
> + */
> + if (id_variant > map_id_variant)
> + return 0;
> +
> + if (id_variant == map_id_variant && id_revision >= map_id_revision)
> + return 0;
> +
> + /*
> + * variant is less than mapfile variant or variants are the same but
> + * the revision doesn't match. Return no match.
> + */
> + return 1;
> +}
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