| 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. |