diff -urNp --exclude CVS --exclude BitKeeper x-ref/arch/i386/kernel/process.c x/arch/i386/kernel/process.c
--- x-ref/arch/i386/kernel/process.c	2003-09-23 03:41:08.000000000 +0200
+++ x/arch/i386/kernel/process.c	2003-09-23 03:41:09.000000000 +0200
@@ -145,11 +145,54 @@ void cpu_idle (void)
 	}
 }
 
+/*
+ * This is a kind of hybrid between poll and halt idle routines. This uses new 
+ * Monitor/Mwait instructions on P4 processors with PNI. We Monitor 
+ * need_resched and go to optimized wait state through Mwait. 
+ * Whenever someone changes need_resched, we would be woken up from Mwait 
+ * (without an IPI).
+ */
+static void mwait_idle (void)
+{
+	int oldval;
+
+	__sti();
+	/* Setting need_resched to -1 skips sending IPI during idle resched */
+	oldval = xchg(&current->need_resched, -1);
+	if (!oldval) {
+		do {
+			__monitor((void *)&current->need_resched, 0, 0);
+			if (current->need_resched != -1)
+				break;
+			__mwait(0, 0);
+			
+		} while (current->need_resched == -1);
+	}
+}
+
+int __init select_idle_routine(void)
+{
+	struct cpuinfo_x86 *c = cpu_data;
+
+	if (cpu_has(c, X86_FEATURE_MWAIT)) {
+		printk("Monitor/Mwait feature present.\n");
+		if (!pm_idle) {
+			pm_idle = mwait_idle;
+		}
+		return 1;
+	}
+	pm_idle = default_idle;
+	return 1;
+}
+
 static int __init idle_setup (char *str)
 {
 	if (!strncmp(str, "poll", 4)) {
 		printk("using polling idle threads.\n");
 		pm_idle = poll_idle;
+	} else if (!strncmp(str, "halt", 4)) {
+		printk("using halt in idle threads.\n");
+		pm_idle = default_idle;
 	}
 
 	return 1;
diff -urNp --exclude CVS --exclude BitKeeper x-ref/arch/i386/kernel/setup.c x/arch/i386/kernel/setup.c
--- x-ref/arch/i386/kernel/setup.c	2003-09-23 03:41:06.000000000 +0200
+++ x/arch/i386/kernel/setup.c	2003-09-23 03:41:09.000000000 +0200
@@ -2293,6 +2293,8 @@ static struct _cache_table cache_table[]
 	{ 0x00, 0, 0}
 };
 
+int select_idle_routine(void);
+
 static void __init init_intel(struct cpuinfo_x86 *c)
 {
 	unsigned int trace = 0, l1i = 0, l1d = 0, l2 = 0, l3 = 0; /* Cache sizes */
@@ -2316,6 +2318,7 @@ static void __init init_intel(struct cpu
 	}
 #endif /* CONFIG_X86_F00F_WORKS_OK */
 
+	select_idle_routine();
 	if (c->cpuid_level > 1) {
 		/* supports eax=2  call */
 		int i, j, n;
@@ -3024,6 +3027,12 @@ static int show_cpuinfo(struct seq_file 
 		NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL,
 		NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL,
 		NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL,
+
+		/* Intel Defined (cpuid 1 and ecx) */
+		NULL, NULL, NULL, "monitor-mwait", NULL, NULL, NULL, NULL,
+		NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL,
+		NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL,
+		NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL,
 	};
 	struct cpuinfo_x86 *c = v;
 	int i, n = c - cpu_data;
diff -urNp --exclude CVS --exclude BitKeeper x-ref/include/asm-i386/cpufeature.h x/include/asm-i386/cpufeature.h
--- x-ref/include/asm-i386/cpufeature.h	2003-09-23 03:41:06.000000000 +0200
+++ x/include/asm-i386/cpufeature.h	2003-09-23 03:42:56.000000000 +0200
@@ -71,6 +71,7 @@
 #define X86_FEATURE_P4		(3*32+ 7) /* P4 */
 
 /* Intel-defined CPU features, CPUID level 0x00000001 (ecx), word 4 */
+#define X86_FEATURE_MWAIT 	(4*32+ 3) /* Monitor-Mwait feature present */
 #define X86_FEATURE_EST		(4*32+ 7) /* Enhanced SpeedStep */
 
 /* VIA/Cyrix/Centaur-defined CPU features, CPUID level 0xC0000001, word 5 */
diff -urNp --exclude CVS --exclude BitKeeper x-ref/include/asm-i386/processor.h x/include/asm-i386/processor.h
--- x-ref/include/asm-i386/processor.h	2003-09-23 03:41:07.000000000 +0200
+++ x/include/asm-i386/processor.h	2003-09-23 03:41:09.000000000 +0200
@@ -485,4 +485,21 @@ static inline void rep_nop(void)
 
 #define ARCH_HAS_SMP_BALANCE
 
+static __inline__ void __monitor(const void *eax, unsigned long ecx, 
+		unsigned long edx)
+{
+	/* "monitor %eax,%ecx,%edx;" */
+	asm volatile(
+		".byte 0x0f,0x01,0xc8;"
+		: :"a" (eax), "c" (ecx), "d"(edx));
+}
+
+static __inline__ void __mwait(unsigned long eax, unsigned long ecx)
+{
+	/* "mwait %eax,%ecx;" */
+	asm volatile(
+		".byte 0x0f,0x01,0xc9;"
+		: :"a" (eax), "c" (ecx));
+}
+  
 #endif /* __ASM_I386_PROCESSOR_H */