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Commit 7eb903f4 authored by Andi Kleen's avatar Andi Kleen Committed by Linus Torvalds
Browse files

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



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

Signed-off-by: default avatarAndi Kleen <ak@suse.de>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent f62a91f6

Documentation/x86_64/cpu-hotplug-spec

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Firmware support for CPU hotplug under Linux/x86-64

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Linux/x86-64 supports CPU hotplug now. For various reasons Linux wants to

know in advance boot time the maximum number of CPUs that could be plugged

into the system. ACPI 3.0 currently has no official way to supply

this information from the firmware to the operating system.



In ACPI each CPU needs an LAPIC object in the MADT table (5.2.11.5 in the

ACPI 3.0 specification).  ACPI already has the concept of disabled LAPIC

objects by setting the Enabled bit in the LAPIC object to zero.



For CPU hotplug Linux/x86-64 expects now that any possible future hotpluggable

CPU is already available in the MADT. If the CPU is not available yet

it should have its LAPIC Enabled bit set to 0. Linux will use the number

of disabled LAPICs to compute the maximum number of future CPUs.



In the worst case the user can overwrite this choice using a command line

option (additional_cpus=...), but it is recommended to supply the correct

number (or a reasonable approximation of it, with erring towards more not less)

in the MADT to avoid manual configuration.