Commit 7eb903f4a5c35c8310f0aa7b0e94aae0b826d837

Authored by Andi Kleen
Committed by Linus Torvalds
1 parent f62a91f691

[PATCH] x86_64: Add documentation for CPU hotplug ACPI extension

Cc: len.brown@intel.com, ashok.ray@intel.com

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

Showing 1 changed file with 21 additions and 0 deletions Side-by-side Diff

Documentation/x86_64/cpu-hotplug-spec
  1 +Firmware support for CPU hotplug under Linux/x86-64
  2 +---------------------------------------------------
  3 +
  4 +Linux/x86-64 supports CPU hotplug now. For various reasons Linux wants to
  5 +know in advance boot time the maximum number of CPUs that could be plugged
  6 +into the system. ACPI 3.0 currently has no official way to supply
  7 +this information from the firmware to the operating system.
  8 +
  9 +In ACPI each CPU needs an LAPIC object in the MADT table (5.2.11.5 in the
  10 +ACPI 3.0 specification). ACPI already has the concept of disabled LAPIC
  11 +objects by setting the Enabled bit in the LAPIC object to zero.
  12 +
  13 +For CPU hotplug Linux/x86-64 expects now that any possible future hotpluggable
  14 +CPU is already available in the MADT. If the CPU is not available yet
  15 +it should have its LAPIC Enabled bit set to 0. Linux will use the number
  16 +of disabled LAPICs to compute the maximum number of future CPUs.
  17 +
  18 +In the worst case the user can overwrite this choice using a command line
  19 +option (additional_cpus=...), but it is recommended to supply the correct
  20 +number (or a reasonable approximation of it, with erring towards more not less)
  21 +in the MADT to avoid manual configuration.