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

[James Clark]

On 14/08/2023 14:07, 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.
> 
> Did you consider adding a raw version of _get_cpuid(), which returns the
> full MIDR just for the purpose of caller strcmp_cpuid_str()?

I did, but I thought that seeing as it would only be used in one place,
and that changing the existing one didn't break anything, that it was
better to not fragment the CPU ID interface. I thought it might also
have repercussions for the other architectures as well. It would also
mean that the MIDR that's stored in the header wouldn't have the version
information, which if we're starting to do things with that could be bad.

There are already callers of strcmp_cpuid_str() so it's probably best to
keep it using the same get_cpuid() string. Unless there is a reason
_not_ to do it? There isn't really anything that can't be done with it
accepting/returning the full unmasked MIDR. If you want the old
behavior, you just set the version fields to 0, which I've also used in
a later patch and is already done in mapfile.csv

> 
> I can't comment on how it will be called in relation to
> strcmp_cpuid_str(), as I am unsure whether strcmp_cpuid_str() should be
> just in arch arm64 code - see comment on later patch.
> 
>  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.
>>
>> 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;
>> -
>>       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.
>> + *
>> + * 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)
> 
> should we check implementator and other fields as a sanity check?
> 

I'm not sure what we could check them against? There are no existing
sanity checks around the MIDR stuff, it just takes whatever MIDR is in
mapfile.csv and from the system. Unless there is something specific to
this change that would benefit from it? Otherwise it sounds like it
could be a separate additional change to what's already in Perf.

>> +{
>> +    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;
>> +}
> 
> Thanks,
> John
>