When cpu is offlined, it cannot access per cpu variable at all.
This patch avoids using per_cpu variable inside play_dead.

Signed-off-by: Shan Haitao <haitao.shan@intel.com>
Signed-off-by: Wei Gang <gang.wei@intel.com>

diff -r dac86c85222e xen/arch/x86/acpi/cpu_idle.c
--- a/xen/arch/x86/acpi/cpu_idle.c	Fri Feb 18 19:07:11 2011 +0000
+++ b/xen/arch/x86/acpi/cpu_idle.c	Wed Feb 23 17:05:32 2011 +0800
@@ -569,7 +569,8 @@ static void acpi_dead_idle(void)
         {
             case ACPI_CSTATE_EM_FFH:
                 /* Not treat interrupt as break event */
-                mwait_idle_with_hints(cx->address, 0);
+                __monitor((void *)&mwait_wakeup(smp_processor_id()), 0, 0);
+                __mwait(cx->address, 0);
                 break;
             case ACPI_CSTATE_EM_SYSIO:
                 inb(cx->address);