1*4882a593Smuzhiyun.. SPDX-License-Identifier: GPL-2.0 2*4882a593Smuzhiyun 3*4882a593Smuzhiyun=================================================== 4*4882a593SmuzhiyunFirmware support for CPU hotplug under Linux/x86-64 5*4882a593Smuzhiyun=================================================== 6*4882a593Smuzhiyun 7*4882a593SmuzhiyunLinux/x86-64 supports CPU hotplug now. For various reasons Linux wants to 8*4882a593Smuzhiyunknow in advance of boot time the maximum number of CPUs that could be plugged 9*4882a593Smuzhiyuninto the system. ACPI 3.0 currently has no official way to supply 10*4882a593Smuzhiyunthis information from the firmware to the operating system. 11*4882a593Smuzhiyun 12*4882a593SmuzhiyunIn ACPI each CPU needs an LAPIC object in the MADT table (5.2.11.5 in the 13*4882a593SmuzhiyunACPI 3.0 specification). ACPI already has the concept of disabled LAPIC 14*4882a593Smuzhiyunobjects by setting the Enabled bit in the LAPIC object to zero. 15*4882a593Smuzhiyun 16*4882a593SmuzhiyunFor CPU hotplug Linux/x86-64 expects now that any possible future hotpluggable 17*4882a593SmuzhiyunCPU is already available in the MADT. If the CPU is not available yet 18*4882a593Smuzhiyunit should have its LAPIC Enabled bit set to 0. Linux will use the number 19*4882a593Smuzhiyunof disabled LAPICs to compute the maximum number of future CPUs. 20*4882a593Smuzhiyun 21*4882a593SmuzhiyunIn the worst case the user can overwrite this choice using a command line 22*4882a593Smuzhiyunoption (additional_cpus=...), but it is recommended to supply the correct 23*4882a593Smuzhiyunnumber (or a reasonable approximation of it, with erring towards more not less) 24*4882a593Smuzhiyunin the MADT to avoid manual configuration. 25