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docs/specs/acpi_cpu_hotplug: Remove legacy cpu hotplug descriptions

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Legacy cpu hotplug has been removed totally and machines start with
modern cpu hotplug interface directly.

Therefore, update the documentation to describe current QEMU cpu hotplug
logic.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20260108033051.777361-7-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Zhao Liu
2026-01-08 11:30:30 +08:00
committed by Paolo Bonzini
parent 067ab16cf7
commit cd531feb36
1 changed files with 3 additions and 25 deletions
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28
docs/specs/acpi_cpu_hotplug.rst
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@@ -8,22 +8,6 @@ ACPI BIOS GPE.2 handler is dedicated for notifying OS about CPU hot-add
and hot-remove events.
Legacy ACPI CPU hotplug interface registers
-------------------------------------------
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.
- The first DWORD in bitmap is used in write mode to switch from legacy
to modern CPU hotplug interface, write 0 into it to do switch.
QEMU sets corresponding CPU bit on hot-add event and issues SCI
with GPE.2 event set. CPU present map is read by ACPI BIOS GPE.2 handler
to notify OS about CPU hot-add events. CPU hot-remove isn't supported.
Modern ACPI CPU hotplug interface registers
-------------------------------------------
@@ -189,20 +173,14 @@ Typical usecases
(x86) Detecting and enabling modern CPU hotplug interface
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
QEMU starts with legacy CPU hotplug interface enabled. Detecting and
switching to modern interface is based on the 2 legacy CPU hotplug features:
QEMU starts with modern CPU hotplug interface enabled. Use following steps to
detect modern CPU hotplug interface:
#. Writes into CPU bitmap are ignored.
#. CPU bitmap always has bit #0 set, corresponding to boot CPU.
Use following steps to detect and enable modern CPU hotplug interface:
#. Store 0x0 to the 'CPU selector' register, attempting to switch to modern mode
#. Store 0x0 to the 'CPU selector' register, to ensure valid selector value
#. Store 0x0 to the 'Command field' register
#. Read the 'Command data 2' register.
If read value is 0x0, the modern interface is enabled.
Otherwise legacy or no CPU hotplug interface available
Otherwise no CPU hotplug interface available
Get a cpu with pending event
^^^^^^^^^^^^^^^^^^^^^^^^^^^^