diff -r 2e8ad0c35792 xen/arch/x86/smpboot.c
--- a/xen/arch/x86/smpboot.c	Thu Sep 18 10:43:08 2008 +0100
+++ b/xen/arch/x86/smpboot.c	Mon Sep 22 18:13:41 2008 +0800
@@ -1310,6 +1310,14 @@
 	return err;
 }
 
+long cpu_down_helper(void *data)
+{
+    int cpu = (unsigned long)data;
+
+    cpu_down(cpu);
+    return 0;
+}
+
 int cpu_up(unsigned int cpu)
 {
 	int err = 0;
diff -r 2e8ad0c35792 xen/arch/x86/sysctl.c
--- a/xen/arch/x86/sysctl.c	Thu Sep 18 10:43:08 2008 +0100
+++ b/xen/arch/x86/sysctl.c	Mon Sep 22 18:13:41 2008 +0800
@@ -92,6 +92,25 @@
     }
     break;
     
+    case XEN_SYSCTL_cpu_hotplug:
+    {
+        uint64_t ops = sysctl->u.cpu_hotplug.ops;
+        unsigned long cpu = sysctl->u.cpu_hotplug.cpu;
+
+        switch ( ops )
+        {
+        case CPU_HOTPLUG_ONLINE:
+            cpu_up(cpu);
+            break;
+        case CPU_HOTPLUG_OFFLINE:
+            continue_hypercall_on_cpu(0, cpu_down_helper, (void *)cpu);
+            break;
+        default:
+            ret = -EINVAL;
+            break;
+        }
+    }
+    break;
 
     default:
         ret = -ENOSYS;
diff -r 2e8ad0c35792 xen/include/asm-x86/smp.h
--- a/xen/include/asm-x86/smp.h	Thu Sep 18 10:43:08 2008 +0100
+++ b/xen/include/asm-x86/smp.h	Mon Sep 22 18:13:41 2008 +0800
@@ -56,6 +56,7 @@
 
 #ifdef CONFIG_HOTPLUG_CPU
 #define cpu_is_offline(cpu) unlikely(!cpu_online(cpu))
+extern long cpu_down_helper(void *data);
 extern int cpu_down(unsigned int cpu);
 extern int cpu_up(unsigned int cpu);
 extern void cpu_exit_clear(void);
diff -r 2e8ad0c35792 xen/include/public/sysctl.h
--- a/xen/include/public/sysctl.h	Thu Sep 18 10:43:08 2008 +0100
+++ b/xen/include/public/sysctl.h	Mon Sep 22 18:13:41 2008 +0800
@@ -262,6 +262,18 @@
 typedef struct xen_sysctl_get_pmstat xen_sysctl_get_pmstat_t;
 DEFINE_XEN_GUEST_HANDLE(xen_sysctl_get_pmstat_t);
 
+#define XEN_SYSCTL_cpu_hotplug       11
+struct xen_sysctl_cpu_hotplug {
+    /* IN variables */
+    uint32_t cpu;   /* Physical cpu. */
+#define CPU_HOTPLUG_ONLINE            0
+#define CPU_HOTPLUG_OFFLINE           1
+    uint64_t ops;   /* hotplug opscode */
+};
+typedef struct xen_sysctl_cpu_hotplug xen_sysctl_cpu_hotplug_t;
+DEFINE_XEN_GUEST_HANDLE(xen_sysctl_cpu_hotplug_t);
+
+
 struct xen_sysctl {
     uint32_t cmd;
     uint32_t interface_version; /* XEN_SYSCTL_INTERFACE_VERSION */
@@ -276,6 +288,7 @@
         struct xen_sysctl_getcpuinfo        getcpuinfo;
         struct xen_sysctl_availheap         availheap;
         struct xen_sysctl_get_pmstat        get_pmstat;
+        struct xen_sysctl_cpu_hotplug       cpu_hotplug;
         uint8_t                             pad[128];
     } u;
 };