linux/Documentation/x86/x86_64/cpu-hotplug-spec
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   1Firmware support for CPU hotplug under Linux/x86-64
   2---------------------------------------------------
   3
   4Linux/x86-64 supports CPU hotplug now. For various reasons Linux wants to
   5know in advance of boot time the maximum number of CPUs that could be plugged
   6into the system. ACPI 3.0 currently has no official way to supply
   7this information from the firmware to the operating system.
   8
   9In ACPI each CPU needs an LAPIC object in the MADT table (5.2.11.5 in the
  10ACPI 3.0 specification).  ACPI already has the concept of disabled LAPIC
  11objects by setting the Enabled bit in the LAPIC object to zero.
  12
  13For CPU hotplug Linux/x86-64 expects now that any possible future hotpluggable
  14CPU is already available in the MADT. If the CPU is not available yet
  15it should have its LAPIC Enabled bit set to 0. Linux will use the number
  16of disabled LAPICs to compute the maximum number of future CPUs.
  17
  18In the worst case the user can overwrite this choice using a command line
  19option (additional_cpus=...), but it is recommended to supply the correct
  20number (or a reasonable approximation of it, with erring towards more not less)
  21in the MADT to avoid manual configuration.
  22
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