4 files changed, 107 insertions, 0 deletions

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 036e89ab470c..b6cfa5f62529 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1727,6 +1727,21 @@ config BOOTPARAM_HOTPLUG_CPU0
           You still can enable the CPU0 hotplug feature at boot by kernel
           parameter cpu0_hotplug.
 
+config DEBUG_HOTPLUG_CPU0
+        def_bool n
+        prompt "Debug CPU0 hotplug"
+        depends on HOTPLUG_CPU && EXPERIMENTAL
+        ---help---
+          Enabling this option offlines CPU0 (if CPU0 can be offlined) as
+          soon as possible and boots up userspace with CPU0 offlined. User
+          can online CPU0 back after boot time.
+
+          To debug CPU0 hotplug, you need to enable CPU0 offline/online
+          feature by either turning on CONFIG_BOOTPARAM_HOTPLUG_CPU0 during
+          compilation or giving cpu0_hotplug kernel parameter at boot.
+
+          If unsure, say N.
+
 config COMPAT_VDSO
         def_bool y
         prompt "Compat VDSO support"
diff --git a/arch/x86/include/asm/cpu.h b/arch/x86/include/asm/cpu.h
index a1195726e8c2..5f9a1243190e 100644
--- a/arch/x86/include/asm/cpu.h
+++ b/arch/x86/include/asm/cpu.h
@@ -29,6 +29,9 @@ struct x86_cpu {
 extern int arch_register_cpu(int num);
 extern void arch_unregister_cpu(int);
 extern void __cpuinit start_cpu0(void);
+#ifdef CONFIG_DEBUG_HOTPLUG_CPU0
+extern int _debug_hotplug_cpu(int cpu, int action);
+#endif
 #endif
 
 DECLARE_PER_CPU(int, cpu_state);
diff --git a/arch/x86/kernel/topology.c b/arch/x86/kernel/topology.c
index 0e7b4a7a7fb8..6e60b5fe2244 100644
--- a/arch/x86/kernel/topology.c
+++ b/arch/x86/kernel/topology.c
@@ -50,6 +50,57 @@ static int __init enable_cpu0_hotplug(char *str)
 __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);
diff --git a/arch/x86/power/cpu.c b/arch/x86/power/cpu.c
index adde77588e25..120cee1c3f8d 100644
--- a/arch/x86/power/cpu.c
+++ b/arch/x86/power/cpu.c
@@ -21,6 +21,7 @@
 #include <asm/suspend.h>
 #include <asm/debugreg.h>
 #include <asm/fpu-internal.h> /* pcntxt_mask */
+#include <asm/cpu.h>
 
 #ifdef CONFIG_X86_32
 static struct saved_context saved_context;
@@ -263,6 +264,43 @@ static int bsp_pm_callback(struct notifier_block *nb, unsigned long action,
         case PM_HIBERNATION_PREPARE:
                 ret = bsp_check();
                 break;
+#ifdef CONFIG_DEBUG_HOTPLUG_CPU0
+        case PM_RESTORE_PREPARE:
+                /*
+                 * When system resumes from hibernation, online CPU0 because
+                 * 1. it's required for resume and
+                 * 2. the CPU was online before hibernation
+                 */
+                if (!cpu_online(0))
+                        _debug_hotplug_cpu(0, 1);
+                break;
+        case PM_POST_RESTORE:
+                /*
+                 * When a resume really happens, this code won't be called.
+                 *
+                 * This code is called only when user space hibernation software
+                 * prepares for snapshot device during boot time. So we just
+                 * call _debug_hotplug_cpu() to restore to CPU0's state prior to
+                 * preparing the snapshot device.
+                 *
+                 * This works for normal boot case in our CPU0 hotplug debug
+                 * mode, i.e. CPU0 is offline and user mode hibernation
+                 * software initializes during boot time.
+                 *
+                 * If CPU0 is online and user application accesses snapshot
+                 * device after boot time, this will offline CPU0 and user may
+                 * see different CPU0 state before and after accessing
+                 * the snapshot device. But hopefully this is not a case when
+                 * user debugging CPU0 hotplug. Even if users hit this case,
+                 * they can easily online CPU0 back.
+                 *
+                 * To simplify this debug code, we only consider normal boot
+                 * case. Otherwise we need to remember CPU0's state and restore
+                 * to that state and resolve racy conditions etc.
+                 */
+                _debug_hotplug_cpu(0, 0);
+                break;
+#endif
         default:
                 break;
         }