doc: Add x86 CPU0 online/offline feature

If CONFIG_BOOTPARAM_HOTPLUG_CPU0 is turned on, CPU0 is hotpluggable. Otherwise,
by default CPU0 is not hotpluggable and kernel parameter cpu0_hotplug enables
CPU0 online/offline feature.

The documentations point out two known CPU0 dependencies. First, resume from
hibernate or suspend always starts from CPU0. So hibernate and suspend are
prevented if CPU0 is offline. Another dependency is PIC interrupts always go
to CPU0.

It's said that some machines may depend on CPU0 to poweroff/reboot. But I
haven't seen such dependency on a few tested machines.

Please let me know if you see any CPU0 dependencies on your machine.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/1352835171-3958-2-git-send-email-fenghua.yu@intel.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 9776f068306b..f7cbe1d98cb4 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1984,6 +1984,20 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 
         nox2apic        [X86-64,APIC] Do not enable x2APIC mode.
 
+        cpu0_hotplug    [X86] Turn on CPU0 hotplug feature when
+                        CONFIG_BOOTPARAM_HOTPLUG_CPU0 is off.
+                        Some features depend on CPU0. Known dependencies are:
+                        1. Resume from suspend/hibernate depends on CPU0.
+                        Suspend/hibernate will fail if CPU0 is offline and you
+                        need to online CPU0 before suspend/hibernate.
+                        2. PIC interrupts also depend on CPU0. CPU0 can't be
+                        removed if a PIC interrupt is detected.
+                        It's said poweroff/reboot may depend on CPU0 on some
+                        machines although I haven't seen such issues so far
+                        after CPU0 is offline on a few tested machines.
+                        If the dependencies are under your control, you can
+                        turn on cpu0_hotplug.
+
         nptcg=          [IA-64] Override max number of concurrent global TLB
                         purges which is reported from either PAL_VM_SUMMARY or
                         SAL PALO.