Re: [PATCH v5 2/6] perf arm64: Allow version comparisons of CPU IDs

[John Garry]

On 16/08/2023 10:12, James Clark wrote:
> On 15/08/2023 10:35, John Garry wrote:
> > 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@xxxxxxx>
> > > ---
> > >    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()
>
> Ah yes, now if all the files fail to open or read then buf will be
> uninitialized. I make it so that it will return EINVAL unless the
> fgets() succeeds, but I don't think we need to add the strtoul() back in?

Right, I don't think that the strtoul() is required.

> > > -
> > >        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?
>
> I don't think that's best for consistency. All the other CPU ID
> comparison functions return the strcmp style return values which is the
> reverse of booleans. We could change them all to bool, but it would be a
> big change, and they'd still have strcmp in the name which suggests
> -1/0/1 return values (although -1 is never used here).
>
> I will add to the comment that 1 is returned for a comparison failure
> thought. That is missing.

ok, fine.

> > > + *
> > > + * 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;
> > > +}