Re: [PATCH v2] xen/arm, xen/common: Add Kconfig option to control Dom0 boot

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.07.2025 19:07, Oleksii Moisieiev wrote:
> This commit introduces a new Kconfig option, `CONFIG_DOM0_BOOT`, to
> allow for building Xen without support for booting a regular domain (Dom0).
> This functionality is primarily intended for the ARM architecture.
> 
> A new Kconfig symbol, `HAS_DOM0`, has been added and is selected by
> default for ARM and X86 architecture. This symbol signifies that an
> architecture has the capability to support a Dom0.
> 
> The `DOM0_BOOT` option depends on `HAS_DOM0` and defaults to 'y'. For
> expert users, this option can be disabled (`CONFIG_EXPERT=y` and no
> `CONFIG_DOM0_BOOT` in the config), which will compile out the Dom0
> creation code on ARM. This is useful for embedded or dom0less-only
> scenarios to reduce binary size and complexity.
> 
> The ARM boot path has been updated to panic if it detects a non-dom0less
> configuration while `CONFIG_DOM0_BOOT` is disabled, preventing an invalid
> boot.
> 
> Signed-off-by: Oleksii Moisieiev <oleksii_moisieiev@xxxxxxxx>
> 
> ---
> 
> Changes in v2:
> - decided not to rename HAS_DOM0 (HAS_OPTIONAL_DOM0 was another option
> suggested in ML) because in this case HAS_DOM0LESS should be renamed
> either.
> - fix order of HAS_DOM0 config parameter
> - add HAS_DOM0 option to x86 architecture.

I question the way changes were made. I don't see the value of ...

> --- a/xen/arch/x86/Kconfig
> +++ b/xen/arch/x86/Kconfig
> @@ -18,6 +18,7 @@ config X86
>       select HAS_COMPAT
>       select HAS_CPUFREQ
>       select HAS_DIT
> +     select HAS_DOM0
>       select HAS_EHCI
>       select HAS_EX_TABLE
>       select HAS_FAST_MULTIPLY

... this, as it has no effect ...

> --- a/xen/common/Kconfig
> +++ b/xen/common/Kconfig
> @@ -21,6 +21,14 @@ config DOM0LESS_BOOT
>         Xen boot without the need of a control domain (Dom0), which could be
>         present anyway.
>  
> +config DOM0_BOOT
> +     bool "Dom0 boot support" if EXPERT
> +     depends on HAS_DOM0 && HAS_DEVICE_TREE && DOMAIN_BUILD_HELPERS

... here due to the further dependencies, and ..,

> +     default y

... hence this option still doesn't end up constant-Y for x86.

Jan