[PATCH v9 5/6] kho: kexec-metadata: track previous kernel chain

[Pratyush Yadav]

On Mon, Mar 16 2026, Breno Leitao wrote:

> 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.
>
> Acked-by: SeongJae Park <sj at kernel.org>
> Reviewed-by: Mike Rapoport (Microsoft) <rppt at kernel.org>
> Signed-off-by: Breno Leitao <leitao at debian.org>

Reviewed-by: Pratyush Yadav <pratyush at kernel.org>

[...]

-- 
Regards,
Pratyush Yadav