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Diffstat (limited to 'Documentation/x86/x86_64/cpu-hotplug-spec.rst')
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diff --git a/Documentation/x86/x86_64/cpu-hotplug-spec.rst b/Documentation/x86/x86_64/cpu-hotplug-spec.rst new file mode 100644 index 000000000000..8d1c91f0c880 --- /dev/null +++ b/Documentation/x86/x86_64/cpu-hotplug-spec.rst @@ -0,0 +1,24 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=================================================== +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. |