This patch series is to refine the memory barriers for AUX ring buffer.

Patches 01 ~ 04 to address the barriers usage in the kernel.  The first
patch is to make clear comment for how to use the barriers between the
data store and aux_head store, this asks the driver to make sure the
data is visible.  Patches 02 ~ 04 is to refine the drivers for barriers
after the data store.

Patch 05 is to use WRITE_ONCE() for updating aux_tail.

Patches 06 ~ 09 is to drop the legacy __sync functions, and polish for
duplicate code and cleanup the build and feature test after
SYNC_COMPARE_AND_SWAP is not used.

Patch 10 introduces a new global variable to indicate the kernel runs in
64-bit mode which can be used to confirm if in compat mode; patch 11
introduces variant functions for accessing AUX head/tail, it resolves
the aotmicity for reading head pointer, and returns error for the tail
is bigger than 4GB.

Have testes the patches on Arm64 Juno platform.

Changes from v3:
- Removed the inapprocate paragraph in the commit log for patch "perf
  auxtrace: Drop legacy __sync functions" (Adrian);
- Added new patch to remove feature-sync-compare-and-swap test (Adrian);
- Th patch for "perf auxtrace: Use WRITE_ONCE() for updating aux_tail",
  is a standlone and simple change, so moved it ahead in the patch set
  for better ordering;
- Minor improvement for commit logs in the last two patches.

Changes from v2:
- Removed auxtrace_mmap__read_snapshot_head(), which has the duplicated
  code with auxtrace_mmap__read_head();
- Cleanuped the build for HAVE_SYNC_COMPARE_AND_SWAP_SUPPORT (Adrian);
- Added global variable "kernel_is_64_bit" (Adrian);
- Added compat variants compat_auxtrace_mmap__{read_head|write_tail}
  (Adrian).


Leo Yan (11):
  perf/ring_buffer: Add comment for barriers on AUX ring buffer
  coresight: tmc-etr: Add barrier after updating AUX ring buffer
  coresight: tmc-etf: Add comment for store ordering
  perf/x86: Add barrier after updating bts
  perf auxtrace: Use WRITE_ONCE() for updating aux_tail
  perf auxtrace: Drop legacy __sync functions
  perf auxtrace: Remove auxtrace_mmap__read_snapshot_head()
  perf: Cleanup for HAVE_SYNC_COMPARE_AND_SWAP_SUPPORT
  tools: Remove feature-sync-compare-and-swap feature detection
  perf env: Set flag for kernel is 64-bit mode
  perf auxtrace: Add compat_auxtrace_mmap__{read_head|write_tail}

 arch/x86/events/intel/bts.c                   |   3 +
 .../hwtracing/coresight/coresight-tmc-etf.c   |   6 +
 .../hwtracing/coresight/coresight-tmc-etr.c   |   8 ++
 kernel/events/ring_buffer.c                   |   9 ++
 tools/build/Makefile.feature                  |   1 -
 tools/build/feature/Makefile                  |   4 -
 tools/build/feature/test-all.c                |   4 -
 .../feature/test-sync-compare-and-swap.c      |  15 ---
 tools/perf/Makefile.config                    |   4 -
 tools/perf/util/auxtrace.c                    |  19 ++-
 tools/perf/util/auxtrace.h                    | 109 ++++++++++++++----
 tools/perf/util/env.c                         |  17 ++-
 tools/perf/util/env.h                         |   1 +
 13 files changed, 136 insertions(+), 64 deletions(-)
 delete mode 100644 tools/build/feature/test-sync-compare-and-swap.c

-- 
2.25.1


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