1.. SPDX-License-Identifier: GPL-2.0
2
3===================================================
4Firmware support for CPU hotplug under Linux/x86-64
5===================================================
6
7Linux/x86-64 supports CPU hotplug now. For various reasons Linux wants to
8know in advance of boot time the maximum number of CPUs that could be plugged
9into the system. ACPI 3.0 currently has no official way to supply
10this information from the firmware to the operating system.
11
12In ACPI each CPU needs an LAPIC object in the MADT table (5.2.11.5 in the
13ACPI 3.0 specification).  ACPI already has the concept of disabled LAPIC
14objects by setting the Enabled bit in the LAPIC object to zero.
15
16For CPU hotplug Linux/x86-64 expects now that any possible future hotpluggable
17CPU is already available in the MADT. If the CPU is not available yet
18it should have its LAPIC Enabled bit set to 0. Linux will use the number
19of disabled LAPICs to compute the maximum number of future CPUs.
20
21In the worst case the user can overwrite this choice using a command line
22option (additional_cpus=...), but it is recommended to supply the correct
23number (or a reasonable approximation of it, with erring towards more not less)
24in the MADT to avoid manual configuration.
25