perf, x86: P4 PMU -- capture extra nmi on overflow

It was observed that in case of performance nmi an extra
nmi may be issued, capture it otherwise 'unknown' nmi
message will appear.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
---
 arch/x86/kernel/cpu/perf_event_p4.c |    9 +++++++++
 1 file changed, 9 insertions(+)

Index: linux-2.6.tip/arch/x86/kernel/cpu/perf_event_p4.c
===================================================================
--- linux-2.6.tip.orig/arch/x86/kernel/cpu/perf_event_p4.c
+++ linux-2.6.tip/arch/x86/kernel/cpu/perf_event_p4.c
@@ -950,6 +950,15 @@ static int p4_pmu_handle_irq(struct pt_r
 	}
 
 	if (handled) {
+		/*
+		 * p4 quirk: extra NMI might happen so we
+		 * add one more here, the back-to-back
+		 * nmi logic will capture it; if more than
+		 * one counter triggered an extra nmi will
+		 * be captured anyway
+		 */
+		if (handled < 2)
+			handled++;
 		/* p4 quirk: unmask it again */
 		apic_write(APIC_LVTPC, apic_read(APIC_LVTPC) & ~APIC_LVT_MASKED);
 		inc_irq_stat(apic_perf_irqs);