zstd_init(, comp_level = 0) initializes decompression part of API only
hat now consists of zstd_decompress_stream() function.

The perf.data PERF_RECORD_COMPRESSED records are decompressed using
zstd_decompress_stream() function into a linked list of mmaped memory
regions of mmap_comp_len size (struct decomp).

After decompression of one COMPRESSED record its content is iterated and
fetched for usual processing. The mmaped memory regions with
decompressed events are kept in the linked list till the tool process
termination.

When dumping raw records (e.g., perf report -D --header) file offsets of
events from compressed records are printed as zero.

Committer notes:

Since now we have support for processing PERF_RECORD_COMPRESSED, we see
none, in raw form, like we saw in the previous patch commiter notes,
they were decompressed into the usual PERF_RECORD_{FORK,MMAP,COMM,etc}
records, we only see the stats for those PERF_RECORD_COMPRESSED events,
and since I used the file generated in the commiter notes for the
previous patch, there they are, 2 compressed records:

  $ perf report --header-only | grep cmdline
  # cmdline : /home/acme/bin/perf record -z2 sleep 1
  $ perf report -D | grep COMPRESS
        COMPRESSED events:          2
        COMPRESSED events:          0
  $ perf report --stdio
  # To display the perf.data header info, please use --header/--header-only options.
  #
  #
  # Total Lost Samples: 0
  #
  # Samples: 15  of event 'cycles:u'
  # Event count (approx.): 962227
  #
  # Overhead  Command  Shared Object     Symbol
  # ........  .......  ................  ...........................
  #
      46.99%  sleep    libc-2.28.so      [.] _dl_addr
      29.24%  sleep    [unknown]         [k] 0xffffffffaea00a67
      16.45%  sleep    libc-2.28.so      [.] __GI__IO_un_link.part.1
       5.92%  sleep    ld-2.28.so        [.] _dl_setup_hash
       1.40%  sleep    libc-2.28.so      [.] __nanosleep
       0.00%  sleep    [unknown]         [k] 0xffffffffaea00163

  #
  # (Tip: To see callchains in a more compact form: perf report -g folded)
  #
  $
Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com>
Reviewed-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/304b0a59-942c-3fe1-da02-aa749f87108b@linux.intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>