[PATCH 3/6] kho: docs: combine concepts and FDT documentation

[Pratyush Yadav]

On Mon, Jan 05 2026, Mike Rapoport wrote:

> From: "Mike Rapoport (Microsoft)" <rppt at kernel.org>
>
> Currently index.rst in KHO documentation looks empty and sad as it only
> contains links to "Kexec Handover Concepts" and "KHO FDT" chapters.
>
> Inline contents of these chapters into index.rst to provide a single
> coherent chapter describing KHO.
>
> While on it, drop parts of the KHO FDT description that will be superseded
> by addition of KHO ABI documentation.
>
> Signed-off-by: Mike Rapoport (Microsoft) <rppt at kernel.org>
[...]
> diff --git a/Documentation/core-api/kho/index.rst b/Documentation/core-api/kho/index.rst
> index 0c63b0c5c143..03cd9afbdb2e 100644
> --- a/Documentation/core-api/kho/index.rst
> +++ b/Documentation/core-api/kho/index.rst
> @@ -4,10 +4,73 @@
>  Kexec Handover Subsystem
>  ========================
>  
> -.. toctree::
> -   :maxdepth: 1
> +Overview
> +========

I ran a test build and Sphinx complains about:

    Documentation/admin-guide/mm/kho.rst:10: WARNING: undefined label: 'kho-concepts' [ref.ref]
    Documentation/admin-guide/mm/kho.rst:31: WARNING: undefined label: 'kho-finalization-phase' [ref.ref]

I suppose you should add these labels here.

Otherwise LGTM.

>  
> -   concepts
> -   fdt
> +Kexec HandOver (KHO) is a mechanism that allows Linux to preserve memory
> +regions, which could contain serialized system states, across kexec.
>  
> -.. only::  subproject and html
> +KHO uses :ref:`flattened device tree (FDT) <kho_fdt>` to pass information about
> +the preserved state from pre-exec kernel to post-kexec kernel and :ref:`scratch
> +memory regions <kho_scratch>` to ensure integrity of the preserved memory.
> +
> +.. _kho_fdt:
> +
> +KHO FDT
> +=======
> +Every KHO kexec carries a KHO specific flattened device tree (FDT) blob that
> +describes the preserved state. The FDT includes properties describing preserved
> +memory regions and nodes that hold subsystem specific state.
> +
> +The preserved memory regions contain either serialized subsystem states, or
> +in-memory data that shall not be touched across kexec. After KHO, subsystems
> +can retrieve and restore the preserved state from KHO FDT.
> +
> +Subsystems participating in KHO can define their own format for state
> +serialization and preservation.
> +
> +.. _kho_scratch:
> +
> +Scratch Regions
> +===============
> +
> +To boot into kexec, we need to have a physically contiguous memory range that
> +contains no handed over memory. Kexec then places the target kernel and initrd
> +into that region. The new kernel exclusively uses this region for memory
> +allocations before during boot up to the initialization of the page allocator.
> +
> +We guarantee that we always have such regions through the scratch regions: On
> +first boot KHO allocates several physically contiguous memory regions. Since
> +after kexec these regions will be used by early memory allocations, there is a
> +scratch region per NUMA node plus a scratch region to satisfy allocations
> +requests that do not require particular NUMA node assignment.
> +By default, size of the scratch region is calculated based on amount of memory
> +allocated during boot. The ``kho_scratch`` kernel command line option may be
> +used to explicitly define size of the scratch regions.
> +The scratch regions are declared as CMA when page allocator is initialized so
> +that their memory can be used during system lifetime. CMA gives us the
> +guarantee that no handover pages land in that region, because handover pages
> +must be at a static physical memory location and CMA enforces that only
> +movable pages can be located inside.
> +
> +After KHO kexec, we ignore the ``kho_scratch`` kernel command line option and
> +instead reuse the exact same region that was originally allocated. This allows
> +us to recursively execute any amount of KHO kexecs. Because we used this region
> +for boot memory allocations and as target memory for kexec blobs, some parts
> +of that memory region may be reserved. These reservations are irrelevant for
> +the next KHO, because kexec can overwrite even the original kernel.
> +
> +KHO finalization phase
> +======================
> +
> +To enable user space based kexec file loader, the kernel needs to be able to
> +provide the FDT that describes the current kernel's state before
> +performing the actual kexec. The process of generating that FDT is
> +called serialization. When the FDT is generated, some properties
> +of the system may become immutable because they are already written down
> +in the FDT. That state is called the KHO finalization phase.
> +
> +See Also
> +========
> +
> +- :doc:`/admin-guide/mm/kho`
> diff --git a/Documentation/core-api/liveupdate.rst b/Documentation/core-api/liveupdate.rst
> index 7960eb15a81f..e2aba13494cf 100644
> --- a/Documentation/core-api/liveupdate.rst
> +++ b/Documentation/core-api/liveupdate.rst
> @@ -58,4 +58,4 @@ See Also
>  ========
>  
>  - :doc:`Live Update uAPI </userspace-api/liveupdate>`
> -- :doc:`/core-api/kho/concepts`
>
> +- :doc:`/core-api/kho/index`
>
> diff --git a/Documentation/mm/memfd_preservation.rst b/Documentation/mm/memfd_preservation.rst
> index 66e0fb6d5ef0..a8a5b476afd3 100644
> --- a/Documentation/mm/memfd_preservation.rst
> +++ b/Documentation/mm/memfd_preservation.rst
> @@ -20,4 +20,4 @@ See Also
>  ========
>  
>  - :doc:`/core-api/liveupdate`
> -- :doc:`/core-api/kho/concepts`
>
> +- :doc:`/core-api/kho/index`

-- 
Regards,
Pratyush Yadav