[PATCH v6 0/4] kho: history: track previous kernel version and kexec boot count

[Breno Leitao <leitao@debian.org>]

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
=======
	[    0.000000] Linux version 6.19.0-rc3-upstream-00047-ge5d992347849
	...
	[    0.000000] KHO: exec from: 6.19.0-rc4-next-20260107upstream-00004-g3071b0dc4498 (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:

 * eb2266312507 ("x86/boot: Fix page table access in 5-level to 4-level paging transition")
 * 77d48d39e991 ("efistub/tpm: Use ACPI reclaim memory for event log to avoid corruption")
 * 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.

Some bugs manifest only after multiple consecutive kexec reboots.
Tracking the kexec count helps identify these cases (this metric is
already used by live update sub-system).

KHO provides a reliable mechanism to pass information between kernels.
By carrying the previous kernel's release string and kexec count
forward, we can print this context at boot time to aid debugging.

The goal of this feature is to have this information being printed in
early boot, so, users can trace back kernel releases in kexec. Systemd
is not helpful because we cannot assume that the previous kernel has
systemd or even write access to the disk (common when using Linux as
bootloaders)

Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: SeongJae Park <sj@kernel.org>
---
Changes in v6:
- s/UAPI/uABI (Mike)
- Expanded the rename of s/fdt/blob to kho_debugfs_* calls also (Mike)
- Link to v5: https://patch.msgid.link/20260126-kho-v5-0-7cd0f69ab204@debian.org

Changes in v5:
- Add a patch to document the new feature.
- Change kho_add_subtree() to receive the size instad of guessing it.
- reworked some code path to make it more standard.
- Link to v4: https://patch.msgid.link/20260121-kho-v4-1-5c8fe77b6804@debian.org

Changes in v4:
- Squashed everything in a single commit
- Moved from FDT to C structs (Pratyush)
- Usage of subtress intead of FDT directly (Pratyush)
- Renamed a bunch of variables and functions.
- Link to v3: https://patch.msgid.link/20260108-kho-v3-0-b1d6b7a89342@debian.org

Changes in v3:
- Remove the extra CONFIG for this feature.
- Reworded some identifiers, properties and printks.
- Better documented the questions raised during v2.
- Link to v2: https://patch.msgid.link/20260102-kho-v2-0-1747b1a3a1d6@debian.org

Changes from v2 to v1 (RFC)
- Track the number of kexecs since cold boot (Pasha)
- Change the printk() order compared to KHO
- Rewording of the commit summary
- Link to RFC: https://patch.msgid.link/20251230-kho-v1-1-4d795a24da9e@debian.org

---
Breno Leitao (4):
      kho: add size parameter to kho_add_subtree()
      kho: rename fdt parameter to blob in kho_add/remove_subtree()
      kho: kexec-metadata: track previous kernel chain
      kho: document kexec-metadata tracking feature

 Documentation/admin-guide/mm/kho.rst        | 39 ++++++++++++
 include/linux/kexec_handover.h              |  8 +--
 include/linux/kho/abi/kexec_handover.h      | 31 +++++++++
 kernel/liveupdate/kexec_handover.c          | 99 +++++++++++++++++++++++++----
 kernel/liveupdate/kexec_handover_debugfs.c  | 28 ++++----
 kernel/liveupdate/kexec_handover_internal.h | 15 +++--
 kernel/liveupdate/luo_core.c                |  3 +-
 lib/test_kho.c                              |  3 +-
 mm/memblock.c                               |  2 +-
 9 files changed, 191 insertions(+), 37 deletions(-)
---
base-commit: ca3a02fda4da8e2c1cb6baee5d72352e9e2cfaea
change-id: 20251230-kho-7707e8a2ef1e

Best regards,
--  
Breno Leitao <leitao@debian.org>