QEMU<->ACPI BIOS CPU hotplug interface
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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):
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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)
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One bit per CPU. Bit position reflects corresponding CPU APIC ID.
Read-only.

CPU hot-add/remove notification:
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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.