Re: [PATCH v3] xen/arm, xen/common: Add Kconfig option to control Dom0 boot

[Grygorii Strashko <grygorii_strashko@xxxxxxxx>]

On 24.09.25 19:00, 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 v3:
> - rephrase error message when dom0less mode wasn't detected while dom0
> is disabled.
> - rephrase help message for DOM0_BOOT config option
> - update DOM0_BOOT dependencies for X86
>
> 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.
>
> CONFIG_DOM0_BOOT Kconfig option was introduced to make the Dom0
> regular (legacy) domain an optional feature that can be compiled out
> from the Xen hypervisor build.
>
> The primary motivation for this change is to enhance modularity and
> produce a cleaner, more specialized hypervisor binary when a control
> domain is not needed. In many embedded or dedicated systems, Xen is
> used in a "dom0less" configuration where guests are pre-configured and
> launched directly by the hypervisor. In these scenarios, the entire
> subsystem for booting and managing Dom0 is unnecessary.
>
> This approach aligns with software quality standards like MISRA C,
> which advocate for the removal of unreachable or unnecessary code to
> improve safety and maintainability. Specifically, this change helps adhere to:
>
> MISRA C:2012, Rule 2.2: "There shall be no dead code"
>
> In a build configured for a dom0less environment, the code responsible
> for creating Dom0 would be considered "dead code" as it would never be
> executed. By using the preprocessor to remove it before compilation,
> we ensure that the final executable is free from this unreachable
> code. This simplifies static analysis, reduces the attack surface,
> and makes the codebase easier to verify, which is critical for
> systems requiring high levels of safety and security.
>
> ---
>   xen/arch/arm/Kconfig        |  1 +
>   xen/arch/arm/domain_build.c |  8 ++++++++
>   xen/arch/arm/setup.c        | 14 ++++++++++----
>   xen/arch/x86/Kconfig        |  1 +
>   xen/common/Kconfig          | 11 +++++++++++
>   5 files changed, 31 insertions(+), 4 deletions(-)
>
> diff --git a/xen/arch/arm/Kconfig b/xen/arch/arm/Kconfig
> index cf6af68299..336b2ed242 100644
> --- a/xen/arch/arm/Kconfig
> +++ b/xen/arch/arm/Kconfig
> @@ -17,6 +17,7 @@ config ARM
>         select GENERIC_UART_INIT
>         select HAS_ALTERNATIVE if HAS_VMAP
>         select HAS_DEVICE_TREE_DISCOVERY
> +       select HAS_DOM0
Here HAS_DOM0 is selected and all dependencies also selected ( 
HAS_DEVICE_TREE_DISCOVERY, DOMAIN_BUILD_HELPERS)

>         select HAS_DOM0LESS
>         select HAS_GRANT_CACHE_FLUSH if GRANT_TABLE
>         select HAS_STACK_PROTECTOR
> diff --git a/xen/arch/arm/domain_build.c b/xen/arch/arm/domain_build.c
> index fb8fbb1650..62602afc78 100644
> --- a/xen/arch/arm/domain_build.c
> +++ b/xen/arch/arm/domain_build.c
> @@ -42,8 +42,10 @@
>   #include <asm/grant_table.h>
>   #include <xen/serial.h>
>   
> +#ifdef CONFIG_DOM0_BOOT
>   static unsigned int __initdata opt_dom0_max_vcpus;
>   integer_param("dom0_max_vcpus", opt_dom0_max_vcpus);
> +#endif
>   
>   /*
>    * If true, the extended regions support is enabled for dom0 and
> @@ -104,6 +106,7 @@ int __init parse_arch_dom0_param(const char *s, const char 
> *e)
>    */
>   #define DOM0_FDT_EXTRA_SIZE (128 + sizeof(struct fdt_reserve_entry))
>   
> +#ifdef CONFIG_DOM0_BOOT
>   unsigned int __init dom0_max_vcpus(void)
>   {
>       if ( opt_dom0_max_vcpus == 0 )
> @@ -116,6 +119,7 @@ unsigned int __init dom0_max_vcpus(void)
>   
>       return opt_dom0_max_vcpus;
>   }
> +#endif
>   
>   /*
>    * Insert the given pages into a memory bank, banks are ordered by address.
> @@ -1962,6 +1966,7 @@ int __init construct_domain(struct domain *d, struct 
> kernel_info *kinfo)
>       return 0;
>   }
>   
> +#ifdef CONFIG_DOM0_BOOT
>   static int __init construct_dom0(struct domain *d)
>   {
>       struct kernel_info kinfo = KERNEL_INFO_INIT;
> @@ -1993,6 +1998,7 @@ static int __init construct_dom0(struct domain *d)
>   
>       return construct_hwdom(&kinfo, NULL);
>   }
> +#endif
>   
>   int __init construct_hwdom(struct kernel_info *kinfo,
>                              const struct dt_device_node *node)
> @@ -2046,6 +2052,7 @@ int __init construct_hwdom(struct kernel_info *kinfo,
>       return construct_domain(d, kinfo);
>   }
>   
> +#ifdef CONFIG_DOM0_BOOT
>   void __init create_dom0(void)
>   {
>       struct domain *dom0;
> @@ -2103,6 +2110,7 @@ void __init create_dom0(void)
>   
>       set_xs_domain(dom0);
>   }
> +#endif /* CONFIG_DOM0_BOOT */
>   
>   /*
>    * Local variables:
> diff --git a/xen/arch/arm/setup.c b/xen/arch/arm/setup.c
> index 7ad870e382..fbb290df60 100644
> --- a/xen/arch/arm/setup.c
> +++ b/xen/arch/arm/setup.c
> @@ -481,12 +481,18 @@ void asmlinkage __init noreturn start_xen(unsigned long 
> fdt_paddr)
>       enable_errata_workarounds();
>       enable_cpu_features();
>   
> -    /* Create initial domain 0. */
> -    if ( !is_dom0less_mode() )
> +    if ( IS_ENABLED(CONFIG_DOM0_BOOT) && !is_dom0less_mode() )
> +    {
> +        /* Create initial domain 0. */
>           create_dom0();
> +    }
>       else
> -        printk(XENLOG_INFO "Xen dom0less mode detected\n");
> -
> +    {
> +        if ( is_dom0less_mode())
> +            printk(XENLOG_INFO "Xen dom0less mode detected\n");
> +        else
> +            panic("Neither dom0 nor dom0less mode was detected, aborting\n");
> +    }
>       if ( acpi_disabled )
>       {
>           create_domUs();
> diff --git a/xen/arch/x86/Kconfig b/xen/arch/x86/Kconfig
> index 3f0f3a0f3a..2aeb361c63 100644
> --- 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
> diff --git a/xen/common/Kconfig b/xen/common/Kconfig
> index 76f9ce705f..10a8fc8718 100644
> --- a/xen/common/Kconfig
> +++ b/xen/common/Kconfig
> @@ -26,6 +26,14 @@ 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
> +       default y
> +       depends on (ARM && HAS_DOM0 && HAS_DEVICE_TREE_DISCOVERY && 
> DOMAIN_BUILD_HELPERS) || (X86 && HAS_DOM0)
Here you repeat the same dependencies. In general, if you introduce HAS_DOM0,
which is expected to be selected by ARCH, then all you need is

        depends on HAS_DOM0

It's ARCH decision to select HAS_DOM0, or not.

> +       help
> +         Dom0 boot support enables Xen to boot to the all-powerful domain 
> (Dom0).
> +         If this isn't enabled Xen is expected to boot in dom0less mode only.
"is expected to boot and launching all domains in dom0less/Hyperlaunch mode 
only."?

> +
>   config DOMAIN_BUILD_HELPERS
>         bool
>   
> @@ -125,6 +133,9 @@ config HAS_DEVICE_TREE_DISCOVERY
>         bool
>         select DEVICE_TREE_PARSE
>   
> +config HAS_DOM0
> +       bool
> +
>   config HAS_DOM0LESS
>         bool
>   
--
Best regards,
-grygorii