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@xxxxxxxx>

--- a/xen/arch/x86/cpu/mwait-idle.c
+++ b/xen/arch/x86/cpu/mwait-idle.c
@@ -501,11 +501,14 @@ int __init mwait_idle_init(struct notifi
 		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;