powerpc/oprofile: Handle events that raise an exception without overflowing

Commit 0837e324 fixes a situation on POWER7
where events can roll back if a specualtive event doesn't actually complete.
This can raise a performance monitor exception.  We need to catch this to ensure
that we reset the PMC.  In all cases the PMC will be less than 256 cycles from
overflow.

This patch lifts Anton's fix for the problem in perf and applies it to oprofile
as well.
Signed-off-by: Eric B Munson <emunson@mgebm.net>
Cc: <stable@kernel.org> # as far back as it applies cleanly
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/oprofile/op_model_power4.c

@@ -261,6 +261,28 @@ static int get_kernel(unsigned long pc, unsigned long mmcra)
 	return is_kernel;
 }
 
+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;
+}
+
 static void power4_handle_interrupt(struct pt_regs *regs,
 				    struct op_counter_config *ctr)
 {
@@ -281,7 +303,7 @@ static void power4_handle_interrupt(struct pt_regs *regs,
 
 	for (i = 0; i < cur_cpu_spec->num_pmcs; ++i) {
 		val = classic_ctr_read(i);
-		if (val < 0) {
+		if (pmc_overflow(val)) {
 			if (oprofile_running && ctr[i].enabled) {
 				oprofile_add_ext_sample(pc, regs, i, is_kernel);
 				classic_ctr_write(i, reset_value[i]);