On Wed, 3 Dec 2025 00:28:13 +0800
Zhao Liu <zhao1.liu@intel.com> wrote:
> 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.
>
> Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
> ---
> Changes since v4:
> * New patch.
> ---
> docs/specs/acpi_cpu_hotplug.rst | 28 +++-------------------------
> 1 file changed, 3 insertions(+), 25 deletions(-)
>
> diff --git a/docs/specs/acpi_cpu_hotplug.rst b/docs/specs/acpi_cpu_hotplug.rst
> index 351057c96761..f49678100044 100644
> --- a/docs/specs/acpi_cpu_hotplug.rst
> +++ b/docs/specs/acpi_cpu_hotplug.rst
> @@ -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:
> -
> -#. 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:
> +QEMU starts with modern CPU hotplug interface enabled. Use following steps to
> +detect 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
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^