This patch series fixes a kernel panic that occurs when booting with
both EFI and KHO (Kexec HandOver) enabled.

The issue arises because EFI's `reserve_regions()` clears all memory
regions with `memblock_remove(0, PHYS_ADDR_MAX)` before rebuilding them
from EFI data. This destroys KHO scratch regions that were set up early
during device tree scanning, causing a panic as the kernel has no valid
memory regions for early allocations.

The first patch introduces `is_kho_boot()` to allow early boot
components to reliably detect if the kernel was booted via KHO-enabled
kexec. The existing `kho_is_enabled()` only checks the command line and
doesn't verify if an actual KHO FDT was passed.

The second patch modifies EFI's `reserve_regions()` to selectively
remove only non-KHO memory regions when KHO is active, preserving the
critical scratch regions while still allowing EFI to rebuild its memory
map.

The patchset was developed/tested on arm64.

On a side note, I have noticed that `kho_populate()` calls
`memblock_set_kho_scratch_only()`, but the `kho` cmdline option is
not checked until much later. Therefore, memblock will use only the
scratch regions that were passed from the outgoing kernel, even if the
incoming kernel doesn't explicitly want that. I am not sure if this is
done on purpose, but in any case we can discuss this in another patch,
as it is orthogonal to this one.

Main Changes in v2 (smaller changes can be found in individual patches):
    - Introduce is_kho_boot()
    - Replaced manual loop with for_each_mem_region macro

Evangelos Petrongonas (2):
  kexec: introduce is_kho_boot()
  efi: Support booting with kexec handover (KHO)

 drivers/firmware/efi/efi-init.c | 28 ++++++++++++++++++++++++----
 include/linux/kexec_handover.h  |  6 ++++++
 kernel/kexec_handover.c         | 20 ++++++++++++++++++++
 3 files changed, 50 insertions(+), 4 deletions(-)

-- 
2.47.3



Amazon Web Services Development Center Germany GmbH
Tamara-Danz-Str. 13
10243 Berlin
Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss
Eingetragen am Amtsgericht Charlottenburg unter HRB 257764 B
Sitz: Berlin
Ust-ID: DE 365 538 597

On Tue, Aug 19, 2025 at 11:22:44PM +0000, Evangelos Petrongonas wrote:
> This patch series fixes a kernel panic that occurs when booting with
> both EFI and KHO (Kexec HandOver) enabled.
> 
> The issue arises because EFI's `reserve_regions()` clears all memory
> regions with `memblock_remove(0, PHYS_ADDR_MAX)` before rebuilding them
> from EFI data. This destroys KHO scratch regions that were set up early
> during device tree scanning, causing a panic as the kernel has no valid
> memory regions for early allocations.
> 
> The first patch introduces `is_kho_boot()` to allow early boot
> components to reliably detect if the kernel was booted via KHO-enabled
> kexec. The existing `kho_is_enabled()` only checks the command line and
> doesn't verify if an actual KHO FDT was passed.
> 
> The second patch modifies EFI's `reserve_regions()` to selectively
> remove only non-KHO memory regions when KHO is active, preserving the
> critical scratch regions while still allowing EFI to rebuild its memory
> map.
> 
> The patchset was developed/tested on arm64.
> 
> On a side note, I have noticed that `kho_populate()` calls
> `memblock_set_kho_scratch_only()`, but the `kho` cmdline option is
> not checked until much later. Therefore, memblock will use only the
> scratch regions that were passed from the outgoing kernel, even if the
> incoming kernel doesn't explicitly want that. I am not sure if this is
> done on purpose, but in any case we can discuss this in another patch,
> as it is orthogonal to this one.

kho_populate runs earlier than we parse the command line, so there is an
implicit assumption that we are going through KHO-enabled kexec if FDT was
passed to the new kernel.

I believe the best way is to document that and make it more explicit that
kho command line parameter only affects the "out" part.
 
> Main Changes in v2 (smaller changes can be found in individual patches):
>     - Introduce is_kho_boot()
>     - Replaced manual loop with for_each_mem_region macro
> 
> Evangelos Petrongonas (2):
>   kexec: introduce is_kho_boot()
>   efi: Support booting with kexec handover (KHO)
> 
>  drivers/firmware/efi/efi-init.c | 28 ++++++++++++++++++++++++----
>  include/linux/kexec_handover.h  |  6 ++++++
>  kernel/kexec_handover.c         | 20 ++++++++++++++++++++
>  3 files changed, 50 insertions(+), 4 deletions(-)
> 
> -- 
> 2.47.3

-- 
Sincerely yours,
Mike.