[PATCH] xen/pm_idle: Make pm_idle be default_idle under Xen.

[Konrad Rzeszutek Wilk]

This patch:

commit d91ee5863b71e8c90eaf6035bff3078a85e2e7b5
Author: Len Brown <len.brown at intel.com>
Date:   Fri Apr 1 18:28:35 2011 -0400

    cpuidle: replace xen access to x86 pm_idle and default_idle

    ..scribble on pm_idle and access default_idle,
   have it simply disable_cpuidle() so acpi_idle will not load and
   architecture default HLT will be used.

idea 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
CPU 1
Modules linked in:

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
RSP: e02b:ffff8801d28ddf10  EFLAGS: 00010082
RAX: ffff8801d28dc010 RBX: ffff8801d28ddfd8 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000001
RBP: ffff8801d28ddf10 R08: 0000000000000000 R09: 0000000000000001
R10: 0000000000000001 R11: ffff8801d28ddfd8 R12: ffffffff81b590d0
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff8801dff81000(0000) knlGS:0000000000000000
CS:  e033 DS: 002b ES: 002b CR0: 000000008005003b
CR2: 0000000000000000 CR3: 0000000001a05000 CR4: 0000000000002660
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000000
Process swapper (pid: 0, threadinfo ffff8801d28dc000, task ffff8801d28cae60)
Stack:
 ffff8801d28ddf40 ffffffff8100e2ed ffff8801dff8e390 c136dfe72feab515
 0000000000000000 0000000000000000 ffff8801d28ddf50 ffffffff8149ee78
 0000000000000000 0000000000000000 0000000000000000 0000000000000000
Call Trace:
 [<ffffffff8100e2ed>] cpu_idle+0xae/0xe8
 [<ffffffff8149ee78>] cpu_bringup_and_idle+0xe/0x10
RIP  [<ffffffff81015d1d>] mwait_idle+0x6f/0xb4
 RSP <ffff8801d28ddf10>

In 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 irregardless 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 at canonical.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk at oracle.com>
---
 arch/x86/include/asm/system.h |    1 +
 arch/x86/kernel/process.c     |    8 ++++++++
 arch/x86/xen/setup.c          |    2 +-
 3 files changed, 10 insertions(+), 1 deletions(-)

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 unsigned long arch_align_stack(unsigned long sp);
 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 1f7f8c8..31f47ba 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -404,6 +404,14 @@ void default_idle(void)
 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 46d6d21..79dfb57 100644
--- a/arch/x86/xen/setup.c
+++ b/arch/x86/xen/setup.c
@@ -448,6 +448,6 @@ void __init xen_arch_setup(void)
 #endif
 	disable_cpuidle();
 	boot_option_idle_override = IDLE_HALT;
-
+	WARN_ON(set_pm_idle_to_default());
 	fiddle_vdso();
 }
-- 
1.7.7.3