xen/pm_idle: Make pm_idle be default_idle under Xen.

The idea behind commit d91ee5863b71 ("cpuidle: replace xen access to x86
pm_idle and default_idle") was to have one call - disable_cpuidle()
which would make pm_idle not be molested by other code.  It disallows
cpuidle_idle_call to be set to pm_idle (which is excellent).

But in the select_idle_routine() and idle_setup(), the pm_idle can still
be set to either: amd_e400_idle, mwait_idle or default_idle.  This
depends on some CPU flags (MWAIT) and in AMD case on the type of CPU.

In case of mwait_idle we can hit some instances where the hypervisor
(Amazon EC2 specifically) sets the MWAIT and we get:

  Brought up 2 CPUs
  invalid opcode: 0000 [#1] SMP

  Pid: 0, comm: swapper Not tainted 3.1.0-0.rc6.git0.3.fc16.x86_64 #1
  RIP: e030:[<ffffffff81015d1d>]  [<ffffffff81015d1d>] mwait_idle+0x6f/0xb4
  ...
  Call Trace:
   [<ffffffff8100e2ed>] cpu_idle+0xae/0xe8
   [<ffffffff8149ee78>] cpu_bringup_and_idle+0xe/0x10
  RIP  [<ffffffff81015d1d>] mwait_idle+0x6f/0xb4
   RSP <ffff8801d28ddf10>

In the case of amd_e400_idle we don't get so spectacular crashes, but we
do end up making an MSR which is trapped in the hypervisor, and then
follow it up with a yield hypercall.  Meaning we end up going to
hypervisor twice instead of just once.

The previous behavior before v3.0 was that pm_idle was set to
default_idle regardless of select_idle_routine/idle_setup.

We want to do that, but only for one specific case: Xen.  This patch
does that.

Fixes RH BZ #739499 and Ubuntu #881076
Reported-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/arch/x86/include/asm/system.h b/arch/x86/include/asm/system.h
index c2ff2a1..2d2f01c 100644
--- a/arch/x86/include/asm/system.h
+++ b/arch/x86/include/asm/system.h
@@ -401,6 +401,7 @@
 extern void free_init_pages(char *what, unsigned long begin, unsigned long end);
 
 void default_idle(void);
+bool set_pm_idle_to_default(void);
 
 void stop_this_cpu(void *dummy);
 
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index b9b3b1a..ee5d4fb 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -403,6 +403,14 @@
 EXPORT_SYMBOL(default_idle);
 #endif
 
+bool set_pm_idle_to_default(void)
+{
+	bool ret = !!pm_idle;
+
+	pm_idle = default_idle;
+
+	return ret;
+}
 void stop_this_cpu(void *dummy)
 {
 	local_irq_disable();
diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
index 38d0af4..1093f80 100644
--- a/arch/x86/xen/setup.c
+++ b/arch/x86/xen/setup.c
@@ -410,6 +410,6 @@
 #endif
 	disable_cpuidle();
 	boot_option_idle_override = IDLE_HALT;
-
+	WARN_ON(set_pm_idle_to_default());
 	fiddle_vdso();
 }