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authorAndi Kleen <ak@suse.de>2006-01-11 22:42:39 +0100
committerLinus Torvalds <torvalds@g5.osdl.org>2006-01-11 19:01:11 -0800
commit7eb903f4a5c35c8310f0aa7b0e94aae0b826d837 (patch)
tree39489c4edd583ae398d7987019a4ba7315bdfcee /Documentation/x86_64
parentf62a91f6911479642c0018290d4248ace4287648 (diff)
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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>
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+Firmware support for CPU hotplug under Linux/x86-64
+---------------------------------------------------
+
+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.