Re: [PATCH v4 02/27] hw/i386/pc: Remove PCMachineClass::legacy_cpu_hotplug field

[Igor Mammedov <imammedo@redhat.com>]

On Thu,  8 May 2025 15:35:25 +0200
Philippe Mathieu-Daudé <philmd@linaro.org> wrote:

> The PCMachineClass::legacy_cpu_hotplug boolean was only used
> by the pc-q35-2.6 and pc-i440fx-2.6 machines, which got
> removed. Remove it and simplify build_dsdt(), removing
> build_legacy_cpu_hotplug_aml() altogether.
> 
> Note, this field was added by commit 679dd1a957d ("pc: use
> new CPU hotplug interface since 2.7 machine type"):
> 
>  >  For compatibility reasons PC/Q35 will start with legacy
>  >  CPU hotplug interface by default but with new CPU hotplug
>  >  AML code since 2.7 machine type. That way legacy firmware
>  >  that doesn't use QEMU generated ACPI tables will be
>  >  able to continue using legacy CPU hotplug interface.
>  >
>  >  While new machine type, with firmware supporting QEMU
>  >  provided ACPI tables, will generate new CPU hotplug AML,
>  >  which will switch to new CPU hotplug interface when
>  >  guest OS executes its _INI method on ACPI tables
>  >  loading.  
> 
> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> Reviewed-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
> Reviewed-by: Thomas Huth <thuth@redhat.com>
> Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
> ---
>  include/hw/acpi/cpu_hotplug.h |   3 -
>  include/hw/i386/pc.h          |   3 -
>  hw/acpi/cpu_hotplug.c         | 230 ----------------------------------
>  hw/i386/acpi-build.c          |   4 +-
>  4 files changed, 1 insertion(+), 239 deletions(-)
> 

...

> diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
> index 3fffa4a3328..625889783ec 100644
> --- a/hw/i386/acpi-build.c
> +++ b/hw/i386/acpi-build.c
> @@ -1465,9 +1465,7 @@ build_dsdt(GArray *table_data, BIOSLinker *linker,
>      }
>      aml_append(dsdt, scope);
>  
> -    if (pcmc->legacy_cpu_hotplug) {
> -        build_legacy_cpu_hotplug_aml(dsdt, machine, pm->cpu_hp_io_base);
> -    } else {
> +    {
>          CPUHotplugFeatures opts = {
>              .acpi_1_compatible = true, .has_legacy_cphp = true,
                                           ^^^^^
that still leaves legacy CPU hotplug hardware around, which we should remove
at the same time as legacy AML.

i.e.:
  drop _INI in hw/acpi/cpu.c
  and at the same time replace legacy_acpi_cpu_hotplug_init() at call sites with
  cpu_hotplug_hw_init()
  after that you can safely remove legacy_cpu_hotplug and no longer needed hw/acpi/cpu_hotplug.c && co


>              .smi_path = pm->smi_on_cpuhp ? "\\_SB.PCI0.SMI0.SMIC" : NULL,