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| 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. | ||