Commit 7eb903f4a5c35c8310f0aa7b0e94aae0b826d837
Committed by
Linus Torvalds
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f62a91f691
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[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. |