2 files changed, 20 insertions, 13 deletions

diff --git a/Documentation/cpu-hotplug.txt b/Documentation/cpu-hotplug.txt
index 08c5d04f3086..e05278087ffa 100644
--- a/Documentation/cpu-hotplug.txt
+++ b/Documentation/cpu-hotplug.txt
@@ -44,10 +44,20 @@ 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
+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
+
+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.
+
+
 CPU maps and such
 -----------------
 [More on cpumaps and primitive to manipulate, please check
diff --git a/arch/ia64/kernel/acpi.c b/arch/ia64/kernel/acpi.c
index 34795ede72e0..ecd44bdc8394 100644
--- a/arch/ia64/kernel/acpi.c
+++ b/arch/ia64/kernel/acpi.c
@@ -794,24 +794,21 @@ __init void prefill_possible_map(void)
         int possible, disabled_cpus;
 
         disabled_cpus = total_cpus - available_cpus;
+
         if (additional_cpus == -1) {
-                if (disabled_cpus > 0) {
-                        possible = total_cpus;
+                if (disabled_cpus > 0)
                         additional_cpus = disabled_cpus;
-                }
-                else {
-                        possible = available_cpus;
+                else
                         additional_cpus = 0;
-                }
-        } else {
-                possible = available_cpus + additional_cpus;
-        }
+        }
+
+        possible = available_cpus + additional_cpus;
+
         if (possible > NR_CPUS)
                 possible = NR_CPUS;
 
         printk(KERN_INFO "SMP: Allowing %d CPUs, %d hotplug CPUs\n",
-                possible,
-                max_t(int, additional_cpus, 0));
+                possible, max((possible - available_cpus), 0));
 
         for (i = 0; i < possible; i++)
                 cpu_set(i, cpu_possible_map);