perf docs: Add info on AMD raw event encoding

AMD processors have events with event select codes and unit masks larger
than a byte. The core PMU, for example, uses 12-bit event select codes
split between bits 0-7 and 32-35 of the PERF_CTL MSRs as can be seen
from /sys/bus/event_sources/devices/cpu/format/*.

The Processor Programming Reference (PPR) lists the event codes as
unified 12-bit hexadecimal values instead and the split between the bits
is not apparent to someone who is not aware of the layout of the
PERF_CTL MSRs.

8-bit event select codes continue to work as the layout matches that of
the PERF_CTL MSRs i.e. bits 0-7 for event select and 8-15 for unit mask.

This adds more details in the perf man pages about using
/sys/bus/event_sources/devices/*/format/* for determining the correct
raw event encoding scheme.

E.g. the "op_cache_hit_miss.op_cache_hit" event with code 0x28f and
umask 0x03 can be programmed using its symbolic name as:

  $ sudo perf --debug perf-event-open stat -e op_cache_hit_miss.op_cache_hit sleep 1
  ------------------------------------------------------------
  perf_event_attr:
    type                             4
    size                             128
    config                           0x20000038f
    sample_type                      IDENTIFIER
    read_format                      TOTAL_TIME_ENABLED|TOTAL_TIME_RUNNING
    disabled                         1
    inherit                          1
    enable_on_exec                   1
    exclude_guest                    1
  ------------------------------------------------------------
  [...]

One might use a simple eventsel+umask combination based on what the
current man pages say and incorrectly program the event as:

  $ sudo perf --debug perf-event-open stat -e r0328f sleep 1
  ------------------------------------------------------------
  perf_event_attr:
    type                             4
    size                             128
    config                           0x328f
    sample_type                      IDENTIFIER
    read_format                      TOTAL_TIME_ENABLED|TOTAL_TIME_RUNNING
    disabled                         1
    inherit                          1
    enable_on_exec                   1
    exclude_guest                    1
  ------------------------------------------------------------
  [...]

When it should have been based on the format from sysfs:

  $ cat /sys/bus/event_source/devices/cpu/format/event
  config:0-7,32-35

  $ sudo perf --debug perf-event-open stat -e r20000038f sleep 1
  ------------------------------------------------------------
  perf_event_attr:
    type                             4
    size                             128
    config                           0x20000038f
    sample_type                      IDENTIFIER
    read_format                      TOTAL_TIME_ENABLED|TOTAL_TIME_RUNNING
    disabled                         1
    inherit                          1
    enable_on_exec                   1
    exclude_guest                    1
  ------------------------------------------------------------
  [...]

Reviewed-by: Kajol Jain <kjain@linux.ibm.com>
Signed-off-by: Sandipan Das <sandipan.das@amd.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Ananth Narayan <ananth.narayan@amd.com>
Cc: Kim Phillips <kim.phillips@amd.com>
Cc: Ravi Bangoria <ravi.bangoria@amd.com>
Cc: Robert Richter <rrichter@amd.com>
Cc: Santosh Shukla <santosh.shukla@amd.com>
Link: https://lore.kernel.org/r/20211123084613.243792-1-sandipan.das@amd.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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