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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 of 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.
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