x86/mwait-idle: Graceful probe failure when MWAIT is disabled

When MWAIT is disabled, intel_idle refuses to probe.
But it may mis-lead the user by blaming this on the model number:

intel_idle: does not run on family 6 modesl 79

So defer the check for MWAIT until after the model# white-list check succeeds,
and if the MWAIT check fails, tell the user how to fix it:

intel_idle: Please enable MWAIT in BIOS SETUP

Signed-off-by: Len Brown <len.brown@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
[Linux commit: a4c447533a18ee86e07232d6344ba12b1f9c5077]
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

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

@@ -917,8 +917,7 @@ static const struct idle_cpu idle_cpu_dnv = {
 };
 
 #define ICPU(model, cpu) \
-    { X86_VENDOR_INTEL, 6, model, X86_FEATURE_MONITOR, \
-        &idle_cpu_##cpu}
+	{ X86_VENDOR_INTEL, 6, model, X86_FEATURE_ALWAYS, &idle_cpu_##cpu}
 
 static const struct x86_cpu_id intel_idle_ids[] __initconstrel = {
 	ICPU(0x1a, nehalem),
@@ -1121,6 +1120,11 @@ static int __init mwait_idle_probe(void)
 		return -ENODEV;
 	}
 
+	if (!boot_cpu_has(X86_FEATURE_MONITOR)) {
+		pr_debug(PREFIX "Please enable MWAIT in BIOS SETUP\n");
+		return -ENODEV;
+	}
+
 	if (boot_cpu_data.cpuid_level < CPUID_MWAIT_LEAF)
 		return -ENODEV;