perf events, x86: Work around the Nehalem AAJ80 erratum

On Nehalem CPUs the retired branch-misses event can be completely bogus,
when there are no branch-misses occuring. When there are a lot of branch
misses then the count is pretty accurate. Still, this leaves us with an
event that over-counts a lot.

Detect this erratum and work it around by using BR_MISP_EXEC.ANY events.
These will also count speculated branches but still it's a lot more
precise in practice than the architectural event.

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/n/tip-yyfg0bxo9jsqxd6a0ovfny27@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 14 insertions, 2 deletions

diff --git a/arch/x86/kernel/cpu/perf_event_intel.c b/arch/x86/kernel/cpu/perf_event_intel.c
index 9194b0698d63..9ae4a2aa7398 100644
--- a/arch/x86/kernel/cpu/perf_event_intel.c
+++ b/arch/x86/kernel/cpu/perf_event_intel.c
@@ -25,7 +25,7 @@ struct intel_percore {
 /*
  * Intel PerfMon, used on Core and later.
  */
-static const u64 intel_perfmon_event_map[] =
+static u64 intel_perfmon_event_map[PERF_COUNT_HW_MAX] __read_mostly =
 {
   [PERF_COUNT_HW_CPU_CYCLES]            = 0x003c,
   [PERF_COUNT_HW_INSTRUCTIONS]          = 0x00c0,
@@ -1308,7 +1308,7 @@ static void intel_clovertown_quirks(void)
          * AJ106 could possibly be worked around by not allowing LBR
          *       usage from PEBS, including the fixup.
          * AJ68  could possibly be worked around by always programming
-         *       a pebs_event_reset[0] value and coping with the lost events.
+         *       a pebs_event_reset[0] value and coping with the lost events.
          *
          * But taken together it might just make sense to not enable PEBS on
          * these chips.
@@ -1412,6 +1412,18 @@ static __init int intel_pmu_init(void)
                 x86_pmu.percore_constraints = intel_nehalem_percore_constraints;
                 x86_pmu.enable_all = intel_pmu_nhm_enable_all;
                 x86_pmu.extra_regs = intel_nehalem_extra_regs;
+
+                if (ebx & 0x40) {
+                        /*
+                         * Erratum AAJ80 detected, we work it around by using
+                         * the BR_MISP_EXEC.ANY event. This will over-count
+                         * branch-misses, but it's still much better than the
+                         * architectural event which is often completely bogus:
+                         */
+                        intel_perfmon_event_map[PERF_COUNT_HW_BRANCH_MISSES] = 0x7f89;
+
+                        pr_cont("erratum AAJ80 worked around, ");
+                }
                 pr_cont("Nehalem events, ");
                 break;