x86: remove mwait capability C-state check

Vegard Nossum reports:

| powertop shows between 200-400 wakeups/second with the description
| "<kernel IPI>: Rescheduling interrupts" when all processors have load (e.g.
| I need to run two busy-loops on my 2-CPU system for this to show up).
|
| The bisect resulted in this commit:
|
| commit 0c07ee38c9d4eb081758f5ad14bbffa7197e1aec
| Date:   Wed Jan 30 13:33:16 2008 +0100
|
|     x86: use the correct cpuid method to detect MWAIT support for C states

remove the functional effects of this patch and make mwait unconditional.

A future patch will turn off mwait on specific CPUs where that causes
power to be wasted.

Bisected-by: Vegard Nossum <vegard.nossum@gmail.com>
Tested-by: Vegard Nossum <vegard.nossum@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/process.c

@@ -99,15 +99,6 @@ static void mwait_idle(void)
 		local_irq_enable();
 }
 
-
-static int __cpuinit mwait_usable(const struct cpuinfo_x86 *c)
-{
-	if (force_mwait)
-		return 1;
-	/* Any C1 states supported? */
-	return c->cpuid_level >= 5 && ((cpuid_edx(5) >> 4) & 0xf) > 0;
-}
-
 /*
  * On SMP it's slightly faster (but much more power-consuming!)
  * to poll the ->work.need_resched flag instead of waiting for the
@@ -131,7 +122,7 @@ void __cpuinit select_idle_routine(const struct cpuinfo_x86 *c)
 			" performance may degrade.\n");
 	}
 #endif
-	if (cpu_has(c, X86_FEATURE_MWAIT) && mwait_usable(c)) {
+	if (cpu_has(c, X86_FEATURE_MWAIT)) {
 		/*
 		 * Skip, if setup has overridden idle.
 		 * One CPU supports mwait => All CPUs supports mwait