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