[PATCH] NMI watchdog Pentium M support

This adda nmi_watchdog=2 support to the Pentium M processor.  The P-M is
a P6 chip, but it shares some chipset logic with the Pentium 4, so it
requires this workaround to function.

Without this patch, NMI gets stuck after 1 count.  With it, the NMI
fires and breaks me out of UHCI-related hard lockups. 

This patch is basically a modified version of the same patch for
oprofile. See the threaded discussion here:

	http://lkml.org/lkml/2004/2/12/181

arch/i386/kernel/nmi.c

@@ -458,6 +458,13 @@ void nmi_watchdog_tick (struct pt_regs * regs)
 			wrmsr(MSR_P4_IQ_CCCR0, P4_NMI_IQ_CCCR0, 0);
 			apic_write(APIC_LVTPC, APIC_DM_NMI);
 		}
+		else if (nmi_perfctr_msr == MSR_P6_PERFCTR0) {
+			/* Only P6 based Pentium M need to re-unmask
+			 * the apic vector but it doesn't hurt
+			 * other P6 variant */
+			apic_write(APIC_LVTPC,
+				   apic_read(APIC_LVTPC) & ~APIC_LVT_MASKED);
+		}
 		wrmsr(nmi_perfctr_msr, -(cpu_khz/nmi_hz*1000), -1);
 	}
 }