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>

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

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);
 		}
 	}