1 files changed, 24 insertions, 3 deletions

diff --git a/Documentation/cpu-hotplug.txt b/Documentation/cpu-hotplug.txt
index 08c5d04f3086..57a09f99ecb0 100644
--- a/Documentation/cpu-hotplug.txt
+++ b/Documentation/cpu-hotplug.txt
@@ -11,6 +11,8 @@
                         Joel Schopp <jschopp@austin.ibm.com>
                 ia64/x86_64:
                         Ashok Raj <ashok.raj@intel.com>
+                s390:
+                        Heiko Carstens <heiko.carstens@de.ibm.com>
 
 Authors: Ashok Raj <ashok.raj@intel.com>
 Lots of feedback: Nathan Lynch <nathanl@austin.ibm.com>,
@@ -44,9 +46,28 @@ maxcpus=n    Restrict boot time cpus to n. Say if you have 4 cpus, using
              maxcpus=2 will only boot 2. You can choose to bring the
              other cpus later online, read FAQ's for more info.
 
-additional_cpus=n       [x86_64 only] use this to limit hotpluggable cpus.
-                        This option sets
-                        cpu_possible_map = cpu_present_map + additional_cpus
+additional_cpus*=n      Use this to limit hotpluggable cpus. This option sets
+                        cpu_possible_map = cpu_present_map + additional_cpus
+
+(*) Option valid only for following architectures
+- x86_64, ia64, s390
+
+ia64 and x86_64 use the number of disabled local apics in ACPI tables MADT
+to determine the number of potentially hot-pluggable cpus. The implementation
+should only rely on this to count the #of cpus, but *MUST* not rely on the
+apicid values in those tables for disabled apics. In the event BIOS doesnt
+mark such hot-pluggable cpus as disabled entries, one could use this
+parameter "additional_cpus=x" to represent those cpus in the cpu_possible_map.
+
+s390 uses the number of cpus it detects at IPL time to also the number of bits
+in cpu_possible_map. If it is desired to add additional cpus at a later time
+the number should be specified using this option or the possible_cpus option.
+
+possible_cpus=n         [s390 only] use this to set hotpluggable cpus.
+                        This option sets possible_cpus bits in
+                        cpu_possible_map. Thus keeping the numbers of bits set
+                        constant even if the machine gets rebooted.
+                        This option overrides additional_cpus.
 
 CPU maps and such
 -----------------