[PATCH 1/1] efi_loader: require at least 128 KiB of stack space

[Ilias Apalodimas]

On Tue, 17 Feb 2026 at 10:46, Heinrich Schuchardt
<heinrich.schuchardt at canonical.com> wrote:
>
> On 2/17/26 09:18, Ilias Apalodimas wrote:
> > Hi Heinrich,
> >
> > On Tue, 17 Feb 2026 at 09:47, Heinrich Schuchardt
> > <heinrich.schuchardt at canonical.com> wrote:
> >>
> >> The UEFI specification requires at least 128 KiB stack space. Consider this
> >> value as a prerequisite for CONFIG_EFI_LOADER.
> >>
> >> Mention the requirement in the CONFIG_STACK_SPACE description and decribe
> >> that the UEFI sub-system uses CONFIG_STACK_SPACE when defining the memory
> >> map.
> >>
> >> Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt at canonical.com>
> >> ---
> >>   Kconfig                | 8 +++++---
> >>   lib/efi_loader/Kconfig | 2 ++
> >>   2 files changed, 7 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/Kconfig b/Kconfig
> >> index ce25ea24a60..99b3cd62526 100644
> >> --- a/Kconfig
> >> +++ b/Kconfig
> >> @@ -615,9 +615,11 @@ config STACK_SIZE
> >>          default 0x4000 if ARCH_STM32
> >>          default 0x1000000
> >>          help
> >> -         Define Max stack size that can be used by U-Boot. This value is used
> >> -         by the UEFI sub-system. On some boards initrd_high is calculated as
> >> -         base stack pointer minus this stack size.
> >> +         Define Max stack size that can be used by U-Boot. The UEFI sub-system
> >> +         considers this value when setting up the memory map. The UEFI
> >> +         specification requires 128 KiB or more of available stack space. On
> >> +         some boards initrd_high is calculated as base stack pointer minus this
> >> +         stack size.
> >>
> >>   config SYS_MEM_TOP_HIDE
> >>          hex "Exclude some memory from U-Boot / OS information"
> >> diff --git a/lib/efi_loader/Kconfig b/lib/efi_loader/Kconfig
> >> index 13e44be1d06..450ffa61c0a 100644
> >> --- a/lib/efi_loader/Kconfig
> >> +++ b/lib/efi_loader/Kconfig
> >> @@ -15,6 +15,8 @@ config EFI_LOADER
> >>          # We need EFI_STUB_32BIT to be set on x86_32 with EFI_STUB
> >>          depends on !EFI_STUB || !X86 || X86_64 || EFI_STUB_32BIT
> >>          depends on !EFI_APP
> >> +       # The EFI specification requires 128 KiB or more of stack space
> >> +       depends on STACK_SIZE >= 0x20000
> >
> > Won't this disable EFI for ARCH_STM32 devices? Those define the
> > default stack size as 0x4000.
>
> The UEFI specification explicitly requires an enabled MMU.
> The ARMv7-M architecture does not offer it.

There was a discussion about this earlier and we decided to keep it
alive though. IIRC Mark explained why we can keep it enabled.

Thanks
/Ilias
>
> Best regards
>
> Heinrich
>
> >
> > Thanks
> > /Ilias
> >>          default y if !ARM || SYS_CPU = armv7 || SYS_CPU = armv8
> >>          select EFI
> >>          select CHARSET
> >> --
> >> 2.51.0
> >>
>