[PATCH v3 0/4] Make KHO Stateless

[Jason Miu <jasonmiu@google.com>]

This series transitions KHO from an xarray-based metadata tracking system
with serialization to using a radix tree data structure that can be
passed directly to the next kernel.

The key motivations for this change are to:
- Eliminate the need for data serialization before kexec.
- Remove the KHO finalize state.
- Pass preservation metadata more directly to the next kernel via the FDT.

The new approach uses a radix tree to mark preserved pages. A
page's physical address and its order are encoded into a single
value. The tree is composed of multiple levels of page-sized tables,
with leaf nodes being bitmap tables where each set bit represents a
preserved page. The physical address of the radix trees root is passed
in the FDT, allowing the next kernel to reconstruct the preserved
memory map.

This series is broken down into the following patches:

1.  kho: Introduce KHO FDT ABI header:
    Adds a dedicated ABI header to define the FDT interface.

2.  kho: Relocate vmalloc preservation structure to KHO ABI header
    Adds `struct kho_vmalloc` and related structs to the ABI header.

3.  kho: Adopt radix tree for preserved memory tracking:    
    Replaces the xarray-based tracker with the new radix tree
    implementation and increments the ABI version.

4.  kho: Remove finalize state and clients:
    Removes the now-obsolete kho_finalize() function and its usage
    from client code and debugfs.

---

Changelog since v2 [1]:
- The preserved memory tracking data structures is defined in the ABI header.
- Relocate the `kho_vmalloc` struct and its helper macro to the KHO ABI
  header to formalize its status as a stable ABI.
- Export the core radix tree manipulation functions as a public API to
  support broader use by other kernel subsystems.
- Update the documents.
- Inline comments have been cleaned up.

[1] https://lore.kernel.org/lkml/20251020100306.2709352-1-jasonmiu@google.com/

---

Jason Miu (4):
  kho: Introduce KHO FDT ABI header
  kho: Relocate vmalloc preservation structure to KHO ABI header
  kho: Adopt radix tree for preserved memory tracking
  kho: Remove finalize state and clients

 Documentation/admin-guide/mm/kho.rst          |  46 +-
 Documentation/core-api/kho/bindings/kho.yaml  |  43 --
 .../core-api/kho/bindings/sub-fdt.yaml        |  27 -
 Documentation/core-api/kho/concepts.rst       |  28 +-
 Documentation/core-api/kho/fdt.rst            |  84 +--
 Documentation/core-api/kho/index.rst          |   1 +
 Documentation/core-api/kho/radix_tree.rst     |  17 +
 include/linux/kexec_handover.h                |  27 +-
 include/linux/kho/abi/kexec_handover.h        | 268 +++++++
 include/linux/kho/abi/memfd.h                 |   2 +-
 include/linux/kho_radix_tree.h                |  81 ++
 kernel/liveupdate/kexec_handover.c            | 694 +++++++++---------
 kernel/liveupdate/kexec_handover_debugfs.c    |  20 -
 kernel/liveupdate/kexec_handover_internal.h   |   3 -
 kernel/liveupdate/luo_core.c                  |  12 +-
 kernel/liveupdate/luo_flb.c                   |   2 +-
 lib/test_kho.c                                |   1 +
 tools/testing/selftests/kho/init.c            |  20 -
 18 files changed, 765 insertions(+), 611 deletions(-)
 delete mode 100644 Documentation/core-api/kho/bindings/kho.yaml
 delete mode 100644 Documentation/core-api/kho/bindings/sub-fdt.yaml
 create mode 100644 Documentation/core-api/kho/radix_tree.rst
 create mode 100644 include/linux/kho/abi/kexec_handover.h
 create mode 100644 include/linux/kho_radix_tree.h


base-commit: 7d31f578f3230f3b7b33b0930b08f9afd8429817
-- 
2.52.0.223.gf5cc29aaa4-goog