perf/x86/intel/lbr: Optimize v4 LBR unfreezing

In Arch perfmon v4 the GLOBAL_STATUS reset automatically unfreezes
LBRs. So no need to do it manually in the LBR code. Add a check
to skip it.

v2: Move test up to beginning of function.
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: eranian@google.com
Link: http://lkml.kernel.org/r/1431285767-27027-9-git-send-email-andi@firstfloor.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/cpu/perf_event_intel_lbr.c

@@ -140,6 +140,13 @@ static void __intel_pmu_lbr_enable(bool pmi)
 	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
 	u64 debugctl, lbr_select = 0, orig_debugctl;
 
+	/*
+	 * No need to unfreeze manually, as v4 can do that as part
+	 * of the GLOBAL_STATUS ack.
+	 */
+	if (pmi && x86_pmu.version >= 4)
+		return;
+
 	/*
 	 * No need to reprogram LBR_SELECT in a PMI, as it
 	 * did not change.