x86, perf_counter, bts: Do not allow kernel BTS tracing for now

Kernel BTS tracing generates too much data too fast for us to
handle, causing the kernel to hang.

Fail for BTS requests for kernel code.

Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>
Acked-by: Peter Zijlstra <a.p.zjilstra@chello.nl>
LKML-Reference: <20090902140616.901253000@intel.com>
[ This is really a workaround - but we want BTS tracing in .32
  so make sure we dont regress. The lockup should be fixed
  ASAP. ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/cpu/perf_counter.c

@@ -984,8 +984,15 @@ static int __hw_perf_counter_init(struct perf_counter *counter)
 	 * Branch tracing:
 	 */
 	if ((attr->config == PERF_COUNT_HW_BRANCH_INSTRUCTIONS) &&
-	    (hwc->sample_period == 1) && !bts_available())
-		return -EOPNOTSUPP;
+	    (hwc->sample_period == 1)) {
+		/* BTS is not supported by this architecture. */
+		if (!bts_available())
+			return -EOPNOTSUPP;
+
+		/* BTS is currently only allowed for user-mode. */
+		if (hwc->config & ARCH_PERFMON_EVENTSEL_OS)
+			return -EOPNOTSUPP;
+	}
 
 	hwc->config |= config;