Re: [PATCH v3 0/2] kho: history: track previous kernel version and kexec boot count

[SeongJae Park <sj@kernel.org>]

On Thu, 08 Jan 2026 08:40:57 -0800 Breno Leitao <leitao@debian.org> 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
> =======
> 	[    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)

Nit.  The above snippet uses tabs for first two lines, while the third line
uses spaces.  I don't really mind this of course, please feel free to ignore.
I just wanted to prove I did read your cover letter ;)

> 
> 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)

Sounds this feature will be useful!  Thank you for detailed cover letter.

> 
> Signed-off-by: Breno Leitao <leitao@debian.org>

The patches also look good to me.  For the series,

Acked-by: SeongJae Park <sj@kernel.org>

> ---
> Changes from v1 to RFC

Nit.  You mean "from RFC (v1) to v2"?  Again, I don't really mind this trivial
thing, so please feel free to ignore.

> - 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
> 
> ---
> 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


Thanks,
SJ

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