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

[Oleksii Moisieiev <Oleksii_Moisieiev@xxxxxxxx>]

On 25/09/2025 12:22, Grygorii Strashko wrote:
>
>
> 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."?
>
Thanks for the review. Hyperlauch is not merged right now so I don't 
think it's a good idea to introduce new term.
Agree with other statemets above.
>> +
>>   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
>