Re: [PATCH v5] hw/i386/pc: Remove PCMachineClass::legacy_cpu_hotplug field

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 26 Nov 2025 23:37:32 +0800
Zhao Liu <zhao1.liu@intel.com> wrote:

> Hi Igor,
> 
> I find after removing legacy cpu hotplug totally, it seems necessary to
> update docs/specs/acpi_cpu_hotplug.rst as well, since then QEMU doesn't
> have legacy cpu hotplug. Do you agree?
> 
> I also attached my update for that doc. If this change is fine for you,
> I will squash that doc change into another patch that removes
> cpu_hotplug.[h|c] - I split your this patch into several parts
> (following you another previous suggestion [*] to make review easier).
> :)

sure thing, whatever makes it easier to read/review

> 
> [*]: https://lore.kernel.org/qemu-devel/20250509171847.0b505c96@imammedo.users.ipa.redhat.com/
> 
> Thanks,
> Zhao
> ---
> From ff3da76cf7de097c42bdaf64850dc4df65b4bf4a Mon Sep 17 00:00:00 2001
> From: Zhao Liu <zhao1.liu@intel.com>
> Date: Wed, 26 Nov 2025 23:06:09 +0800
> Subject: [PATCH] docs/specs/acpi_cpu_hotplug.rst: remove legacy cpu hotplug
>  descriptions
> 
> 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>

LGTM

> ---
>  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
>  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> --
> 2.34.1
>