From: Breno Leitao Date: Mon, 16 Mar 2026 11:54:36 +0000 (-0700) Subject: kho: document kexec-metadata tracking feature X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=e524feaad5467f39a56d2697f7db31f02796dc7d;p=thirdparty%2Fkernel%2Flinux.git 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. Link: https://lore.kernel.org/20260316-kho-v9-6-ed6dcd951988@debian.org Signed-off-by: Breno Leitao Suggested-by: Mike Rapoport Reviewed-by: Mike Rapoport (Microsoft) Reviewed-by: Pratyush Yadav Cc: Alexander Graf Cc: David Hildenbrand Cc: Jonathan Corbet Cc: "Liam R. Howlett" Cc: Lorenzo Stoakes Cc: Michal Hocko Cc: Pasha Tatashin Cc: SeongJae Park Cc: Shuah Khan Cc: Suren Baghdasaryan Cc: Vlastimil Babka Signed-off-by: Andrew Morton --- diff --git a/Documentation/admin-guide/mm/kho.rst b/Documentation/admin-guide/mm/kho.rst index 6a4ddf344046..2c26e560bd78 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 ==================