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

[dmkhn@xxxxxxxxx]

On Tue, Jul 29, 2025 at 10:20:54AM +0200, Jan Beulich wrote:
> On 28.07.2025 23:28, dmkhn@xxxxxxxxx wrote:
> > On Mon, Jul 28, 2025 at 05:07:30PM +0000, 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>
> >>
> >> ---
> >>
> >> ---
> >>  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(-)
> >
> > I think there should be changes in
> >   include/xen/domain.h
> > and
> >   arch/arm/include/asm/setup.h
> > to compile out declarations of dom0_max_vcpus() and create_dom0() under new
> > CONFIG_DOM0_BOOT.
> 
> Adding #ifdef-ary just to hide declarations is often merely adding clutter,
> without providing a clear benefit. I didn't check in this case, but I think
> when making such a request you want to clarify what the gains would be of
> adding more #ifdef.

re: clutter: fully agree.

I was thinking about this following code where ifdef-ery may be needed:

+    if ( IS_ENABLED(CONFIG_DOM0_BOOT) && !is_dom0less_mode() )
+    {
+        /* Create initial domain 0. */
         create_dom0();
+    }

But looks like compiler is correctly throwing away create_dom0() call.

> 
> Jan