x86/smpboot: Sanitize uniprocessor init

The UP related setups for local apic are mangled into smp_sanity_check().

That results in duplicate calls to disable_smp() and makes the code
hard to follow. Let smp_sanity_check() return dedicated values for the
various exit reasons and handle them at the call site.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Borislav Petkov <bp@alien8.de>
Cc: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Tony Luck <tony.luck@intel.com>
Link: http://lkml.kernel.org/r/20150115211703.987833932@linutronix.deSigned-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/smpboot.c

@@ -990,6 +990,8 @@ void arch_disable_smp_support(void)
  */
 static __init void disable_smp(void)
 {
+	pr_info("SMP disabled\n");
+
 	disable_ioapic_support();
 
 	init_cpu_present(cpumask_of(0));
@@ -1003,6 +1005,13 @@ static __init void disable_smp(void)
 	cpumask_set_cpu(0, cpu_core_mask(0));
 }
 
+enum {
+	SMP_OK,
+	SMP_NO_CONFIG,
+	SMP_NO_APIC,
+	SMP_FORCE_UP,
+};
+
 /*
  * Various sanity checks.
  */
@@ -1050,10 +1059,7 @@ static int __init smp_sanity_check(unsigned max_cpus)
 	if (!smp_found_config && !acpi_lapic) {
 		preempt_enable();
 		pr_notice("SMP motherboard not detected\n");
-		disable_smp();
-		if (APIC_init_uniprocessor())
-			pr_notice("Local APIC not detected. Using dummy APIC emulation.\n");
-		return -1;
+		return SMP_NO_CONFIG;
 	}
 
 	/*
@@ -1077,7 +1083,7 @@ static int __init smp_sanity_check(unsigned max_cpus)
 				boot_cpu_physical_apicid);
 			pr_err("... forcing use of dummy APIC emulation (tell your hw vendor)\n");
 		}
-		return -1;
+		return SMP_NO_APIC;
 	}
 
 	verify_local_APIC();
@@ -1087,12 +1093,10 @@ static int __init smp_sanity_check(unsigned max_cpus)
 	 */
 	if (!max_cpus) {
 		pr_info("SMP mode deactivated\n");
-		disable_ioapic_support();
-		apic_bsp_setup();
-		return -1;
+		return SMP_FORCE_UP;
 	}
 
-	return 0;
+	return SMP_OK;
 }
 
 static void __init smp_cpu_index_default(void)
@@ -1132,10 +1136,21 @@ void __init native_smp_prepare_cpus(unsigned int max_cpus)
 	}
 	set_cpu_sibling_map(0);
 
-	if (smp_sanity_check(max_cpus) < 0) {
-		pr_info("SMP disabled\n");
+	switch (smp_sanity_check(max_cpus)) {
+	case SMP_NO_CONFIG:
 		disable_smp();
+		if (APIC_init_uniprocessor())
+			pr_notice("Local APIC not detected. Using dummy APIC emulation.\n");
+		return;
+	case SMP_NO_APIC:
+		disable_smp();
+		return;
+	case SMP_FORCE_UP:
+		disable_smp();
+		apic_bsp_setup();
 		return;
+	case SMP_OK:
+		break;
 	}
 
 	default_setup_apic_routing();