perf, powerpc: Handle events that raise an exception without overflowing

Events on POWER7 can roll back if a speculative event doesn't
eventually complete. Unfortunately in some rare cases they will
raise a performance monitor exception. We need to catch this to
ensure we reset the PMC. In all cases the PMC will be 256 or less
cycles from overflow.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: <stable@kernel.org> # as far back as it applies cleanly
LKML-Reference: <20110309143842.6c22845e@kryten>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 24 insertions, 1 deletions

diff --git a/arch/powerpc/include/asm/reg.h b/arch/powerpc/include/asm/reg.h
index 125fc1ad665d..7626fa78e1f8 100644
--- a/arch/powerpc/include/asm/reg.h
+++ b/arch/powerpc/include/asm/reg.h
@@ -880,6 +880,7 @@
 #define PV_970          0x0039
 #define PV_POWER5       0x003A
 #define PV_POWER5p      0x003B
+#define PV_POWER7       0x003F
 #define PV_970FX        0x003C
 #define PV_630          0x0040
 #define PV_630p 0x0041
diff --git a/arch/powerpc/kernel/perf_event.c b/arch/powerpc/kernel/perf_event.c
index ab6f6beadb57..97e0ae414940 100644
--- a/arch/powerpc/kernel/perf_event.c
+++ b/arch/powerpc/kernel/perf_event.c
@@ -1269,6 +1269,28 @@ unsigned long perf_instruction_pointer(struct pt_regs *regs)
         return ip;
 }
 
+static bool pmc_overflow(unsigned long val)
+{
+        if ((int)val < 0)
+                return true;
+
+        /*
+         * Events on POWER7 can roll back if a speculative event doesn't
+         * eventually complete. Unfortunately in some rare cases they will
+         * raise a performance monitor exception. We need to catch this to
+         * ensure we reset the PMC. In all cases the PMC will be 256 or less
+         * cycles from overflow.
+         *
+         * We only do this if the first pass fails to find any overflowing
+         * PMCs because a user might set a period of less than 256 and we
+         * don't want to mistakenly reset them.
+         */
+        if (__is_processor(PV_POWER7) && ((0x80000000 - val) <= 256))
+                return true;
+
+        return false;
+}
+
 /*
  * Performance monitor interrupt stuff
  */
@@ -1316,7 +1338,7 @@ static void perf_event_interrupt(struct pt_regs *regs)
                         if (is_limited_pmc(i + 1))
                                 continue;
                         val = read_pmc(i + 1);
-                        if ((int)val < 0)
+                        if (pmc_overflow(val))
                                 write_pmc(i + 1, 0);
                 }
         }