[PATCH v4 1/7] x86/topology.c: Support functions for BSP online/offline

["Fenghua Yu" <fenghua.yu@intel.com>]

From: Fenghua Yu <fenghua.yu@intel.com>

By default, BSP can't be hotpluggable because bsp_hotpluggable is 0. Kernel
parameter bsp_hotplug can enable BSP hotplug feature.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
---
 arch/x86/kernel/topology.c |   24 +++++++++++++++++-------
 1 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/topology.c b/arch/x86/kernel/topology.c
index 76ee977..4493868 100644
--- a/arch/x86/kernel/topology.c
+++ b/arch/x86/kernel/topology.c
@@ -35,18 +35,28 @@
 static DEFINE_PER_CPU(struct x86_cpu, cpu_devices);
 
 #ifdef CONFIG_HOTPLUG_CPU
+
+static int __read_mostly bsp_hotpluggable;
+
+static int __init enable_bsp_hotplug(char *str)
+{
+	bsp_hotpluggable = 1;
+	return 1;
+}
+
+__setup("bsp_hotplug", enable_bsp_hotplug);
+
 int __ref arch_register_cpu(int num)
 {
 	/*
-	 * 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.
+	 * 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 || bsp_hotpluggable)
 		per_cpu(cpu_devices, num).cpu.hotpluggable = 1;
 
 	return register_cpu(&per_cpu(cpu_devices, num).cpu, num);
-- 
1.6.0.3