x86, topology: Debug CPU0 hotplug

CONFIG_DEBUG_HOTPLUG_CPU0 is for debugging the CPU0 hotplug feature. The switch
offlines CPU0 as soon as possible and boots userspace up with CPU0 offlined.
User can online CPU0 back after boot time. The default value of the switch is
off.

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.

It's safe and early place to take down CPU0 after all hotplug notifiers
are installed and SMP is booted.

Please note that some applications or drivers, e.g. some versions of udevd,
during boot time may put CPU0 online again in this CPU0 hotplug debug mode.

In this debug mode, setup_local_APIC() may report a warning on max_loops<=0
when CPU0 is onlined back after boot time. This is because pending interrupt in
IRR can not move to ISR. The warning is not CPU0 specfic and it can happen on
other CPUs as well. It is harmless except the first CPU0 online takes a bit
longer time. And so this debug mode is useful to expose this issue. I'll send
a seperate patch to fix this generic warning issue.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/1352835171-3958-15-git-send-email-fenghua.yu@intel.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

1 files changed, 15 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"