1 files changed, 94 insertions, 7 deletions

diff --git a/arch/x86/kernel/topology.c b/arch/x86/kernel/topology.c
index 76ee97709a00..6e60b5fe2244 100644
--- a/arch/x86/kernel/topology.c
+++ b/arch/x86/kernel/topology.c
@@ -30,23 +30,110 @@
 #include <linux/mmzone.h>
 #include <linux/init.h>
 #include <linux/smp.h>
+#include <linux/irq.h>
 #include <asm/cpu.h>
 
 static DEFINE_PER_CPU(struct x86_cpu, cpu_devices);
 
 #ifdef CONFIG_HOTPLUG_CPU
+
+#ifdef CONFIG_BOOTPARAM_HOTPLUG_CPU0
+static int cpu0_hotpluggable = 1;
+#else
+static int cpu0_hotpluggable;
+static int __init enable_cpu0_hotplug(char *str)
+{
+	cpu0_hotpluggable = 1;
+	return 1;
+}
+
+__setup("cpu0_hotplug", enable_cpu0_hotplug);
+#endif
+
+#ifdef CONFIG_DEBUG_HOTPLUG_CPU0
+/*
+ * This function offlines a CPU as early as possible and allows userspace to
+ * boot up without the CPU. The CPU can be onlined back by user after boot.
+ *
+ * This is only called for debugging CPU offline/online feature.
+ */
+int __ref _debug_hotplug_cpu(int cpu, int action)
+{
+	struct device *dev = get_cpu_device(cpu);
+	int ret;
+
+	if (!cpu_is_hotpluggable(cpu))
+		return -EINVAL;
+
+	cpu_hotplug_driver_lock();
+
+	switch (action) {
+	case 0:
+		ret = cpu_down(cpu);
+		if (!ret) {
+			pr_info("CPU %u is now offline\n", cpu);
+			kobject_uevent(&dev->kobj, KOBJ_OFFLINE);
+		} else
+			pr_debug("Can't offline CPU%d.\n", cpu);
+		break;
+	case 1:
+		ret = cpu_up(cpu);
+		if (!ret)
+			kobject_uevent(&dev->kobj, KOBJ_ONLINE);
+		else
+			pr_debug("Can't online CPU%d.\n", cpu);
+		break;
+	default:
+		ret = -EINVAL;
+	}
+
+	cpu_hotplug_driver_unlock();
+
+	return ret;
+}
+
+static int __init debug_hotplug_cpu(void)
+{
+	_debug_hotplug_cpu(0, 0);
+	return 0;
+}
+
+late_initcall_sync(debug_hotplug_cpu);
+#endif /* CONFIG_DEBUG_HOTPLUG_CPU0 */
+
 int __ref arch_register_cpu(int num)
 {
+	struct cpuinfo_x86 *c = &cpu_data(num);
+
+	/*
+	 * Currently CPU0 is only hotpluggable on Intel platforms. Other
+	 * vendors can add hotplug support later.
+	 */
+	if (c->x86_vendor != X86_VENDOR_INTEL)
+		cpu0_hotpluggable = 0;
+
 	/*
-	 * CPU0 cannot be offlined due to several
-	 * restrictions and assumptions in kernel. This basically
-	 * doesn't add a control file, one cannot attempt to offline
-	 * BSP.
+	 * Two known BSP/CPU0 dependencies: Resume from suspend/hibernate
+	 * depends on BSP. PIC interrupts depend on BSP.
 	 *
-	 * Also certain PCI quirks require not to enable hotplug control
-	 * for all CPU's.
+	 * If the BSP depencies are under control, one can tell kernel to
+	 * enable BSP hotplug. This basically adds a control file and
+	 * one can attempt to offline BSP.
 	 */
-	if (num)
+	if (num == 0 && cpu0_hotpluggable) {
+		unsigned int irq;
+		/*
+		 * We won't take down the boot processor on i386 if some
+		 * interrupts only are able to be serviced by the BSP in PIC.
+		 */
+		for_each_active_irq(irq) {
+			if (!IO_APIC_IRQ(irq) && irq_has_action(irq)) {
+				cpu0_hotpluggable = 0;
+				break;
+			}
+		}
+	}
+	if (num || cpu0_hotpluggable)
 		per_cpu(cpu_devices, num).cpu.hotpluggable = 1;
 
 	return register_cpu(&per_cpu(cpu_devices, num).cpu, num);