Merge branch 'x86-bsp-hotplug-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 BSP hotplug changes from Ingo Molnar:
 "This tree enables CPU#0 (the boot processor) to be onlined/offlined on
  x86, just like any other CPU.  Enabled on Intel CPUs for now.

  Allowing this required the identification and fixing of latent CPU#0
  assumptions (such as CPU#0 initializations, etc.) in the x86
  architecture code, plus the identification of barriers to
  BSP-offlining, such as active PIC interrupts which can only be
  serviced on the BSP.

  It's behind a default-off option, and there's a debug option that
  allows the automatic testing of this feature.

  The motivation of this feature is to allow and prepare for true
  CPU-hotplug hardware support: recent changes to MCE support enable us
  to detect a deteriorating but not yet hard-failing L1/L2 cache on a
  CPU that could be soft-unplugged - or a failing L3 cache on a
  multi-socket system.

  Note that true hardware hot-plug is not yet fully enabled by this,
  because that requires a special platform wakeup sequence to be sent to
  the freshly powered up CPU#0.  Future patches for this are planned,
  once such a platform exists.  Chicken and egg"

* 'x86-bsp-hotplug-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86, topology: Debug CPU0 hotplug
  x86/i387.c: Initialize thread xstate only on CPU0 only once
  x86, hotplug: Handle retrigger irq by the first available CPU
  x86, hotplug: The first online processor saves the MTRR state
  x86, hotplug: During CPU0 online, enable x2apic, set_numa_node.
  x86, hotplug: Wake up CPU0 via NMI instead of INIT, SIPI, SIPI
  x86-32, hotplug: Add start_cpu0() entry point to head_32.S
  x86-64, hotplug: Add start_cpu0() entry point to head_64.S
  kernel/cpu.c: Add comment for priority in cpu_hotplug_pm_callback
  x86, hotplug, suspend: Online CPU0 for suspend or hibernate
  x86, hotplug: Support functions for CPU0 online/offline
  x86, topology: Don't offline CPU0 if any PIC irq can not be migrated out of it
  x86, Kconfig: Add config switch for CPU0 hotplug
  doc: Add x86 CPU0 online/offline feature

1 files changed, 44 insertions, 0 deletions

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 6c304438b50..2d643255c40 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1698,6 +1698,50 @@ config HOTPLUG_CPU
 	    automatically on SMP systems. )
 	  Say N if you want to disable CPU hotplug.
 
+config BOOTPARAM_HOTPLUG_CPU0
+	bool "Set default setting of cpu0_hotpluggable"
+	default n
+	depends on HOTPLUG_CPU && EXPERIMENTAL
+	---help---
+	  Set whether default state of cpu0_hotpluggable is on or off.
+
+	  Say Y here to enable CPU0 hotplug by default. If this switch
+	  is turned on, there is no need to give cpu0_hotplug kernel
+	  parameter and the CPU0 hotplug feature is enabled by default.
+
+	  Please note: there are two known CPU0 dependencies if you want
+	  to enable the CPU0 hotplug feature either by this switch or by
+	  cpu0_hotplug kernel parameter.
+
+	  First, resume from hibernate or suspend always starts from CPU0.
+	  So hibernate and suspend are prevented if CPU0 is offline.
+
+	  Second dependency is PIC interrupts always go to CPU0. CPU0 can not
+	  offline if any interrupt can not migrate out of CPU0. There may
+	  be other CPU0 dependencies.
+
+	  Please make sure the dependencies are under your control before
+	  you enable this feature.
+
+	  Say N if you don't want to enable CPU0 hotplug feature by default.
+	  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"