kho: kexec-metadata: track previous kernel chain

Use Kexec Handover (KHO) to pass the previous kernel's version string and
the number of kexec reboots since the last cold boot to the next kernel,
and print it at boot time.

Example output:
    [    0.000000] KHO: exec from: 6.19.0-rc4-next-20260107 (count 1)

Motivation
==========

Bugs that only reproduce when kexecing from specific kernel versions are
difficult to diagnose.  These issues occur when a buggy kernel kexecs into
a new kernel, with the bug manifesting only in the second kernel.

Recent examples include the following commits:

 * commit eb2266312507 ("x86/boot: Fix page table access in
   5-level to 4-level paging transition")
 * commit 77d48d39e991 ("efistub/tpm: Use ACPI reclaim memory
   for event log to avoid corruption")
 * commit 64b45dd46e15 ("x86/efi: skip memattr table on kexec
   boot")

As kexec-based reboots become more common, these version-dependent bugs
are appearing more frequently.  At scale, correlating crashes to the
previous kernel version is challenging, especially when issues only occur
in specific transition scenarios.

Implementation
==============

The kexec metadata is stored as a plain C struct (struct
kho_kexec_metadata) rather than FDT format, for simplicity and direct
field access.  It is registered via kho_add_subtree() as a separate
subtree, keeping it independent from the core KHO ABI.  This design
choice:

 - Keeps the core KHO ABI minimal and stable
 - Allows the metadata format to evolve independently
 - Avoids requiring version bumps for all KHO consumers (LUO, etc.)
   when the metadata format changes

The struct kho_kexec_metadata contains two fields:
 - previous_release: The kernel version that initiated the kexec
 - kexec_count: Number of kexec boots since last cold boot

On cold boot, kexec_count starts at 0 and increments with each kexec.  The
count helps identify issues that only manifest after multiple consecutive
kexec reboots.

[leitao@debian.org: call kho_kexec_metadata_init() for both boot paths]
  Link: https://lore.kernel.org/all/20260309-kho-v8-5-c3abcf4ac750@debian.org/ [1]
  Link: https://lore.kernel.org/20260409-kho_fix_merge_issue-v1-1-710c84ceaa85@debian.org
Link: https://lore.kernel.org/20260316-kho-v9-5-ed6dcd951988@debian.org
Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: SeongJae Park <sj@kernel.org>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
Cc: Alexander Graf <graf@amazon.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

0 files changed, 0 insertions, 0 deletions