162306a36Sopenharmony_ci.. SPDX-License-Identifier: GPL-2.0 262306a36Sopenharmony_ci 362306a36Sopenharmony_ci=================================================== 462306a36Sopenharmony_ciFirmware support for CPU hotplug under Linux/x86-64 562306a36Sopenharmony_ci=================================================== 662306a36Sopenharmony_ci 762306a36Sopenharmony_ciLinux/x86-64 supports CPU hotplug now. For various reasons Linux wants to 862306a36Sopenharmony_ciknow in advance of boot time the maximum number of CPUs that could be plugged 962306a36Sopenharmony_ciinto the system. ACPI 3.0 currently has no official way to supply 1062306a36Sopenharmony_cithis information from the firmware to the operating system. 1162306a36Sopenharmony_ci 1262306a36Sopenharmony_ciIn ACPI each CPU needs an LAPIC object in the MADT table (5.2.11.5 in the 1362306a36Sopenharmony_ciACPI 3.0 specification). ACPI already has the concept of disabled LAPIC 1462306a36Sopenharmony_ciobjects by setting the Enabled bit in the LAPIC object to zero. 1562306a36Sopenharmony_ci 1662306a36Sopenharmony_ciFor CPU hotplug Linux/x86-64 expects now that any possible future hotpluggable 1762306a36Sopenharmony_ciCPU is already available in the MADT. If the CPU is not available yet 1862306a36Sopenharmony_ciit should have its LAPIC Enabled bit set to 0. Linux will use the number 1962306a36Sopenharmony_ciof disabled LAPICs to compute the maximum number of future CPUs. 2062306a36Sopenharmony_ci 2162306a36Sopenharmony_ciIn the worst case the user can overwrite this choice using a command line 2262306a36Sopenharmony_cioption (additional_cpus=...), but it is recommended to supply the correct 2362306a36Sopenharmony_cinumber (or a reasonable approximation of it, with erring towards more not less) 2462306a36Sopenharmony_ciin the MADT to avoid manual configuration. 25