linux-imx

perf/x86/intel/uncore: Add enable_box for client MSR uncore

There are bug reports about miscounting uncore counters on some
client machines like Sandybridge, Broadwell and Skylake. It is
very likely to be observed on idle systems.

This issue is caused by a hardware issue. PERF_GLOBAL_CTL could be
cleared after Package C7, and nothing will be count.
The related errata (HSD 158) could be found in:

  www.intel.com/content/dam/www/public/us/en/documents/specification-updates/4th-gen-core-family-desktop-specification-update.pdf

This patch tries to work around this issue by re-enabling PERF_GLOBAL_CTL
in ->enable_box(). The workaround does not cover all cases. It helps for new
events after returning from C7. But it cannot prevent C7, it will still
miscount if a counter is already active.

There is no drawback in leaving it enabled, so it does not need
disable_box() here.

Signed-off-by: Kan Liang <kan.liang@intel.com>
Cc: <stable@vger.kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Link: http://lkml.kernel.org/r/1470925874-59943-1-git-send-email-kan.liang@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>