linux-kernel - [PATCH v5 4/4] kho: document kexec-metadata tracking feature

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Message-Id: <20260126-kho-v5-4-7cd0f69ab204@debian.org>
Date: Mon, 26 Jan 2026 08:07:26 -0800
From: Breno Leitao <leitao@...ian.org>
To: Alexander Graf <graf@...zon.com>, Mike Rapoport <rppt@...nel.org>, 
 Pasha Tatashin <pasha.tatashin@...een.com>, 
 Pratyush Yadav <pratyush@...nel.org>
Cc: linux-kernel@...r.kernel.org, kexec@...ts.infradead.org, 
 linux-mm@...ck.org, usamaarif642@...il.com, rmikey@...a.com, clm@...com, 
 riel@...riel.com, Breno Leitao <leitao@...ian.org>, 
 SeongJae Park <sj@...nel.org>, kernel-team@...a.com
Subject: [PATCH v5 4/4] kho: document kexec-metadata tracking feature

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@...nel.org>
Signed-off-by: Breno Leitao <leitao@...ian.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 6dc18ed4b8861..3771e764bb513 100644
--- a/Documentation/admin-guide/mm/kho.rst
+++ b/Documentation/admin-guide/mm/kho.rst
@@ -62,6 +62,45 @@ You can move the system out of KHO finalization phase again by calling ::
 After this command, the KHO FDT is no longer available in
 ``/sys/kernel/debug/kho/out/fdt``.
 
+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
 ==================
 

-- 
2.47.3