libperf: Add support for user space counter access

x86 and arm64 can both support direct access of event counters in
userspace. The access sequence is less than trivial and currently exists
in perf test code (tools/perf/arch/x86/tests/rdpmc.c) with copies in
projects such as PAPI and libpfm4.

In order to support userspace access, an event must be mmapped first
with perf_evsel__mmap(). Then subsequent calls to perf_evsel__read()
will use the fast path (assuming the arch supports it).

Committer notes:

Added a '__maybe_unused' attribute to the read_perf_counter() argument
to fix the build on arches other than x86_64 and arm.

Committer testing:

  Building and running the libperf tests in verbose mode (V=1) now shows
  those "loop = N, count = N" extra lines, testing user space counter
  access.

  # make V=1 -C tools/lib/perf tests
  make: Entering directory '/home/acme/git/perf/tools/lib/perf'
  make -f /home/acme/git/perf/tools/build/Makefile.build dir=. obj=libperf
  make -C /home/acme/git/perf/tools/lib/api/ O= libapi.a
  make -f /home/acme/git/perf/tools/build/Makefile.build dir=./fd obj=libapi
  make -f /home/acme/git/perf/tools/build/Makefile.build dir=./fs obj=libapi
  make -C tests
  gcc -I/home/acme/git/perf/tools/lib/perf/include -I/home/acme/git/perf/tools/include -I/home/acme/git/perf/tools/lib -g -Wall -o test-cpumap-a test-cpumap.c ../libperf.a /home/acme/git/perf/tools/lib/api/libapi.a
  gcc -I/home/acme/git/perf/tools/lib/perf/include -I/home/acme/git/perf/tools/include -I/home/acme/git/perf/tools/lib -g -Wall -o test-threadmap-a test-threadmap.c ../libperf.a /home/acme/git/perf/tools/lib/api/libapi.a
  gcc -I/home/acme/git/perf/tools/lib/perf/include -I/home/acme/git/perf/tools/include -I/home/acme/git/perf/tools/lib -g -Wall -o test-evlist-a test-evlist.c ../libperf.a /home/acme/git/perf/tools/lib/api/libapi.a
  gcc -I/home/acme/git/perf/tools/lib/perf/include -I/home/acme/git/perf/tools/include -I/home/acme/git/perf/tools/lib -g -Wall -o test-evsel-a test-evsel.c ../libperf.a /home/acme/git/perf/tools/lib/api/libapi.a
  gcc -I/home/acme/git/perf/tools/lib/perf/include -I/home/acme/git/perf/tools/include -I/home/acme/git/perf/tools/lib -g -Wall -L.. -o test-cpumap-so test-cpumap.c /home/acme/git/perf/tools/lib/api/libapi.a -lperf
  gcc -I/home/acme/git/perf/tools/lib/perf/include -I/home/acme/git/perf/tools/include -I/home/acme/git/perf/tools/lib -g -Wall -L.. -o test-threadmap-so test-threadmap.c /home/acme/git/perf/tools/lib/api/libapi.a -lperf
  gcc -I/home/acme/git/perf/tools/lib/perf/include -I/home/acme/git/perf/tools/include -I/home/acme/git/perf/tools/lib -g -Wall -L.. -o test-evlist-so test-evlist.c /home/acme/git/perf/tools/lib/api/libapi.a -lperf
  gcc -I/home/acme/git/perf/tools/lib/perf/include -I/home/acme/git/perf/tools/include -I/home/acme/git/perf/tools/lib -g -Wall -L.. -o test-evsel-so test-evsel.c /home/acme/git/perf/tools/lib/api/libapi.a -lperf
  make -C tests run
  running static:
  - running test-cpumap.c...OK
  - running test-threadmap.c...OK
  - running test-evlist.c...OK
  - running test-evsel.c...
  	loop = 65536, count = 333926
  	loop = 131072, count = 655781
  	loop = 262144, count = 1311141
  	loop = 524288, count = 2630126
  	loop = 1048576, count = 5256955
  	loop = 65536, count = 524594
  	loop = 131072, count = 1058916
  	loop = 262144, count = 2097458
  	loop = 524288, count = 4205429
  	loop = 1048576, count = 8406606
  OK
  running dynamic:
  - running test-cpumap.c...OK
  - running test-threadmap.c...OK
  - running test-evlist.c...OK
  - running test-evsel.c...
  	loop = 65536, count = 328102
  	loop = 131072, count = 655782
  	loop = 262144, count = 1317494
  	loop = 524288, count = 2627851
  	loop = 1048576, count = 5255187
  	loop = 65536, count = 524601
  	loop = 131072, count = 1048923
  	loop = 262144, count = 2107917
  	loop = 524288, count = 4194606
  	loop = 1048576, count = 8409322
  OK
  make: Leaving directory '/home/acme/git/perf/tools/lib/perf'
  #

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Itaru Kitayama <itaru.kitayama@gmail.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will@kernel.org>
Link: http://lore.kernel.org/lkml/20210414155412.3697605-4-robh@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>