powerpc/perf: Exclude kernel samples while counting events in user space.

Perf event attritube supports exclude_kernel flag to avoid
sampling/profiling in supervisor state (kernel). Based on this event
attr flag, Monitor Mode Control Register bit is set to freeze on
supervisor state. But sometimes (due to hardware limitation), Sampled
Instruction Address Register (SIAR) locks on to kernel address even
when freeze on supervisor is set. Patch here adds a check to drop
those samples.

Cc: stable@vger.kernel.org
Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1606289215-1433-1-git-send-email-atrajeev@linux.vnet.ibm.com

arch/powerpc/perf/core-book3s.c

@@ -2158,6 +2158,16 @@ static void record_and_restart(struct perf_event *event, unsigned long val,
 	local64_set(&event->hw.period_left, left);
 	perf_event_update_userpage(event);
 
+	/*
+	 * Due to hardware limitation, sometimes SIAR could sample a kernel
+	 * address even when freeze on supervisor state (kernel) is set in
+	 * MMCR2. Check attr.exclude_kernel and address to drop the sample in
+	 * these cases.
+	 */
+	if (event->attr.exclude_kernel && record)
+		if (is_kernel_addr(mfspr(SPRN_SIAR)))
+			record = 0;
+
 	/*
 	 * Finally record data if requested.
 	 */