x86: get the MWAIT idle driver in sync with the ACPI one

.. with respect to behavior when there is no HPET broadcast support
(for using the PIT broadcast instead, it requires explicitly enabling
CPU idle management).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>

xen/arch/x86/cpu/mwait-idle.c

@@ -501,11 +501,14 @@ int __init mwait_idle_init(struct notifier_block *nfb)
 		return -ENODEV;
 
 	err = mwait_idle_probe();
-	if (!err) {
-		if (!boot_cpu_has(X86_FEATURE_ARAT))
-			hpet_broadcast_init();
-		if (!lapic_timer_init())
+	if (!err && !boot_cpu_has(X86_FEATURE_ARAT)) {
+		hpet_broadcast_init();
+		if (xen_cpuidle < 0 && !hpet_broadcast_is_available())
+			err = -ENODEV;
+		else if(!lapic_timer_init())
 			err = -EINVAL;
+		if (err)
+			pr_debug(PREFIX "not used (%d)\n", err);
 	}
 	if (!err) {
 		nfb->notifier_call = mwait_idle_cpu_init;