[PATCH v9 6/6] kho: document kexec-metadata tracking feature

Breno Leitao
2026-03-16T11:56:35Z

Add documentation for the kexec-metadata feature that tracks the
previous kernel version and kexec boot count across kexec reboots.
This helps diagnose bugs that only reproduce when kexecing from
specific kernel versions.

Suggested-by: Mike Rapoport <rppt@kernel.org>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
Signed-off-by: Breno Leitao <leitao@debian.org>
---
Documentation/admin-guide/mm/kho.rst | 39 ++++++++++++++++++++++++++++++++++++
1 file changed, 39 insertions(+)

diff --git a/Documentation/admin-guide/mm/kho.rst b/Documentation/admin-guide/mm/kho.rst
index 6a4ddf3440465..2c26e560bd78a 100644
--- a/Documentation/admin-guide/mm/kho.rst
+++ b/Documentation/admin-guide/mm/kho.rst
@@ -42,6 +42,45 @@ For example, if you used ``reserve_mem`` command line parameter to create
an early memory reservation, the new kernel will have that memory at the
same physical address as the old kernel.

+Kexec Metadata
+==============
+
+KHO automatically tracks metadata about the kexec chain, passing information
+about the previous kernel to the next kernel. This feature helps diagnose
+bugs that only reproduce when kexecing from specific kernel versions.
+
+On each KHO kexec, the kernel logs the previous kernel's version and the
+number of kexec reboots since the last cold boot::
+
+ [ 0.000000] KHO: exec from: 6.19.0-rc4-next-20260107 (count 1)
+
+The metadata includes:
+
+``previous_release``
+ The kernel version string (from ``uname -r``) of the kernel that
+ initiated the kexec.
+
+``kexec_count``
+ The number of kexec boots since the last cold boot. On cold boot,
+ this counter starts at 0 and increments with each kexec. This helps
+ identify issues that only manifest after multiple consecutive kexec
+ reboots.
+
+Use Cases
+---------
+
+This metadata is particularly useful for debugging kexec transition bugs,
+where a buggy kernel kexecs into a new kernel and the bug manifests only
+in the second kernel. Examples of such bugs include:
+
+- Memory corruption from the previous kernel affecting the new kernel
+- Incorrect hardware state left by the previous kernel
+- Firmware/ACPI state issues that only appear in kexec scenarios
+
+At scale, correlating crashes to the previous kernel version enables
+faster root cause analysis when issues only occur in specific kernel
+transition scenarios.
+
debugfs Interfaces
==================

--
2.52.0