qemu/docs/specs/acpi_cpu_hotplug.txt

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QEMU<->ACPI BIOS CPU hotplug interface
--------------------------------------
QEMU supports CPU hotplug via ACPI. This document
describes the interface between QEMU and the ACPI BIOS.
ACPI GPE block (IO ports 0xafe0-0xafe3, byte access):
-----------------------------------------
Generic ACPI GPE block. Bit 2 (GPE.2) used to notify CPU
hot-add/remove event to ACPI BIOS, via SCI interrupt.
CPU present bitmap for:
ICH9-LPC (IO port 0x0cd8-0xcf7, 1-byte access)
PIIX-PM (IO port 0xaf00-0xaf1f, 1-byte access)
---------------------------------------------------------------
One bit per CPU. Bit position reflects corresponding CPU APIC ID.
Read-only.
CPU hot-add/remove notification:
-----------------------------------------------------
QEMU sets/clears corresponding CPU bit on hot-add/remove event.
CPU present map read by ACPI BIOS GPE.2 handler to notify OS of CPU
hot-(un)plug events.