Re: [PATCH v5] xen/arm, xen/common: Add Kconfig option to control Dom0 boot

["Orzel, Michal" <michal.orzel@xxxxxxx>]

On 02-Oct-26 15:26, 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>
> Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> ---
> 
> Changes in v5:
> - rebased on top of the latest staging.
> - add Ack
> 
> Changes in v4:
> - change Misra rule to 2.1 from 2.2 in description
> - remove extra dependencies for ARM architecture from DOM0_BOOT
> - rephrase DOM0_BOOT help by adding hyperlaunch
> - DOM0_BOOT is mandatory for x86 architecture
> 
> 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 2.1: "A project shall not contain unreachable code"
But with this patch you will introduce a lot of unreachable code anyway.

> 
> 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        | 13 ++++++++++---
>  xen/arch/x86/Kconfig        |  1 +
>  xen/common/Kconfig          | 12 ++++++++++++
>  5 files changed, 32 insertions(+), 3 deletions(-)
> 
> diff --git a/xen/arch/arm/Kconfig b/xen/arch/arm/Kconfig
> index 06289005f4..6d0936b353 100644
> --- a/xen/arch/arm/Kconfig
> +++ b/xen/arch/arm/Kconfig
> @@ -18,6 +18,7 @@ config ARM
>       select GENERIC_UART_INIT
>       select HAS_ALTERNATIVE if HAS_VMAP
>       select HAS_DEVICE_TREE_DISCOVERY
> +     select HAS_DOM0
>       select HAS_DOM0LESS
>       select HAS_GRANT_CACHE_FLUSH if GRANT_TABLE
>       select HAS_SHARED_INFO
> diff --git a/xen/arch/arm/domain_build.c b/xen/arch/arm/domain_build.c
> index 72d5316180..cc932d7686 100644
> --- a/xen/arch/arm/domain_build.c
> +++ b/xen/arch/arm/domain_build.c
> @@ -43,8 +43,10 @@
>  #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
What about the other dom0-only options: dom0_mem (and CONFIG_DOM0_MEM),
dom0-llc-colors and dom0=sve?

>  
>  /*
>   * If true, the extended regions support is enabled for dom0 and
> @@ -123,6 +125,7 @@ paddr_t __init hwdom_get_fdt_alloc_size(void)
>      return ACPI_DOM0_FDT_MIN_SIZE;
>  }
>  
> +#ifdef CONFIG_DOM0_BOOT
>  unsigned int __init dom0_max_vcpus(void)
>  {
>      if ( opt_dom0_max_vcpus == 0 )
> @@ -135,6 +138,7 @@ 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.
> @@ -1862,6 +1866,7 @@ 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)
This makes dom0_set_llc_colors() unreachable. The same goes for
prepare_acpi() with ACPI=y, as dom0less is DT only. ACPI should be prevented
when DOM0_BOOT=n.

>  {
>      struct kernel_info kinfo = KERNEL_INFO_INIT;
> @@ -1893,6 +1898,7 @@ 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)
> @@ -1944,6 +1950,7 @@ 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;
> @@ -2001,6 +2008,7 @@ 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 16f899dff5..ea9c037fba 100644
> --- a/xen/arch/arm/setup.c
> +++ b/xen/arch/arm/setup.c
> @@ -491,11 +491,18 @@ void asmlinkage __init noreturn start_xen(unsigned long 
> fdt_paddr)
>  
>      do_init_boottests();
>  
> -    /* 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())
Missing space after () and before )

> +            printk(XENLOG_INFO "Xen dom0less mode detected\n");
> +        else
> +            panic("Neither dom0 nor dom0less mode was detected, aborting\n");
> +    }
To avoid calling is_dom0less_mode() twice, how about:

    if ( is_dom0less_mode() )
        printk(XENLOG_INFO "Xen dom0less mode detected\n");
    else if ( IS_ENABLED(CONFIG_DOM0_BOOT) )
        create_dom0();
    else
        panic("Neither dom0 nor dom0less mode was detected, aborting\n");

~Michal