perf, x86: Make a second write to performance counter if needed

On Netburst PMU we need a second write to a performance counter
due to cpu erratum.

A simple flag test instead of alternative instructions was choosen
because wrmsrl is already a macro and if virtualization is turned
on will need an additional wrapper call which is more expencise.

nb: we should propably switch to jump-labels as only this facility
reach the mainline.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Robert Richter <robert.richter@amd.com>
Cc: Lin Ming <ming.m.lin@intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <20100602212304.GC5264@lenovo>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 20 insertions, 1 deletions

diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index af04c6fa59cb..79e199843db6 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -220,6 +220,7 @@ struct x86_pmu {
                                                  struct perf_event *event);
         struct event_constraint *event_constraints;
         void            (*quirks)(void);
+        int             perfctr_second_write;
 
         int             (*cpu_prepare)(int cpu);
         void            (*cpu_starting)(int cpu);
@@ -925,8 +926,17 @@ x86_perf_event_set_period(struct perf_event *event)
          */
         atomic64_set(&hwc->prev_count, (u64)-left);
 
-        wrmsrl(hwc->event_base + idx,
+        wrmsrl(hwc->event_base + idx, (u64)(-left) & x86_pmu.cntval_mask);
+
+        /*
+         * Due to erratum on certan cpu we need
+         * a second write to be sure the register
+         * is updated properly
+         */
+        if (x86_pmu.perfctr_second_write) {
+                wrmsrl(hwc->event_base + idx,
                         (u64)(-left) & x86_pmu.cntval_mask);
+        }
 
         perf_event_update_userpage(event);
 
diff --git a/arch/x86/kernel/cpu/perf_event_p4.c b/arch/x86/kernel/cpu/perf_event_p4.c
index ae85d69644d1..9286e736a70a 100644
--- a/arch/x86/kernel/cpu/perf_event_p4.c
+++ b/arch/x86/kernel/cpu/perf_event_p4.c
@@ -829,6 +829,15 @@ static __initconst const struct x86_pmu p4_pmu = {
         .max_period             = (1ULL << 39) - 1,
         .hw_config              = p4_hw_config,
         .schedule_events        = p4_pmu_schedule_events,
+        /*
+         * This handles erratum N15 in intel doc 249199-029,
+         * the counter may not be updated correctly on write
+         * so we need a second write operation to do the trick
+         * (the official workaround didn't work)
+         *
+         * the former idea is taken from OProfile code
+         */
+        .perfctr_second_write   = 1,
 };
 
 static __init int p4_pmu_init(void)