perf, x86, nmi: Move LVT un-masking into irq handlers

It was noticed that P4 machines were generating double NMIs for
each perf event.  These extra NMIs lead to 'Dazed and confused'
messages on the screen.

I tracked this down to a P4 quirk that said the overflow bit had
to be cleared before re-enabling the apic LVT mask.  My first
attempt was to move the un-masking inside the perf nmi handler
from before the chipset NMI handler to after.

This broke Nehalem boxes that seem to like the unmasking before
the counters themselves are re-enabled.

In order to keep this change simple for 2.6.39, I decided to
just simply move the apic LVT un-masking to the beginning of all
the chipset NMI handlers, with the exception of Pentium4's to
fix the double NMI issue.

Later on we can move the un-masking to later in the handlers to
save a number of 'extra' NMIs on those particular chipsets.

I tested this change on a P4 machine, an AMD machine, a Nehalem
box, and a core2quad box.  'perf top' worked correctly along
with various other small 'perf record' runs.  Anything high
stress breaks all the machines but that is a different problem.

Thanks to various people for testing different versions of this
patch.

Reported-and-tested-by: Shaun Ruffell <sruffell@digium.com>
Signed-off-by: Don Zickus <dzickus@redhat.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Link: http://lkml.kernel.org/r/1303900353-10242-1-git-send-email-dzickus@redhat.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>
CC: Cyrill Gorcunov <gorcunov@gmail.com>

arch/x86/kernel/cpu/perf_event_intel.c

@@ -933,6 +933,16 @@ static int intel_pmu_handle_irq(struct pt_regs *regs)
 
 	cpuc = &__get_cpu_var(cpu_hw_events);
 
+	/*
+	 * Some chipsets need to unmask the LVTPC in a particular spot
+	 * inside the nmi handler.  As a result, the unmasking was pushed
+	 * into all the nmi handlers.
+	 *
+	 * This handler doesn't seem to have any issues with the unmasking
+	 * so it was left at the top.
+	 */
+	apic_write(APIC_LVTPC, APIC_DM_NMI);
+
 	intel_pmu_disable_all();
 	handled = intel_pmu_drain_bts_buffer();
 	status = intel_pmu_get_status();