linux-kernel - Re: [PATCH v4] kho: kexec-metadata: track previous kernel chain

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Message-ID: <2vxzecnca453.fsf@kernel.org>
Date: Mon, 26 Jan 2026 14:35:04 +0100
From: Pratyush Yadav <pratyush@...nel.org>
To: Breno Leitao <leitao@...ian.org>
Cc: Alexander Graf <graf@...zon.com>,  Mike Rapoport <rppt@...nel.org>,
  Pasha Tatashin <pasha.tatashin@...een.com>,  Pratyush Yadav
 <pratyush@...nel.org>,  linux-kernel@...r.kernel.org,
  kexec@...ts.infradead.org,  linux-mm@...ck.org,  usamaarif642@...il.com,
  rmikey@...a.com,  clm@...com,  riel@...riel.com,  kernel-team@...a.com,
  SeongJae Park <sj@...nel.org>
Subject: Re: [PATCH v4] kho: kexec-metadata: track previous kernel chain

Hi Breno,

On Wed, Jan 21 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:
>
>  * 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.
>
> 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_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.
>
> Signed-off-by: Breno Leitao <leitao@...ian.org>
> Acked-by: SeongJae Park <sj@...nel.org>
[...]
> diff --git a/include/linux/kho/abi/kexec_handover.h b/include/linux/kho/abi/kexec_handover.h
> index 285eda8a36e45..e18022a4e664d 100644
> --- a/include/linux/kho/abi/kexec_handover.h
> +++ b/include/linux/kho/abi/kexec_handover.h
> @@ -11,6 +11,7 @@
>  #define _LINUX_KHO_ABI_KEXEC_HANDOVER_H
>  
>  #include <linux/types.h>
> +#include <linux/utsname.h>
>  
>  /**
>   * DOC: Kexec Handover ABI
> @@ -84,6 +85,34 @@
>  /* The FDT property for sub-FDTs. */
>  #define KHO_FDT_SUB_TREE_PROP_NAME "fdt"
>  
> +/**
> + * DOC: Kexec Metadata ABI
> + *
> + * The "kexec-metadata" subtree stores optional metadata about the kexec chain.
> + * It is registered via kho_add_subtree(), keeping it independent from the core
> + * KHO ABI. This allows the metadata format to evolve without affecting other
> + * KHO consumers.
> + *
> + * The metadata is stored as a plain C struct rather than FDT format for
> + * simplicity and direct field access.
> + */
> +
> +/**
> + * struct kho_kexec_metadata - Kexec metadata passed between kernels
> + * @previous_release: Kernel version string that initiated the kexec

Other than Mike's comments, I only have a small nitpick. Please add a
comment here that using __NEW_UTS_LEN in the ABI is safe here since it
is a part of UAPI.

LGTM otherwise.

> + * @kexec_count: Number of kexec boots since last cold boot
> + *
> + * This structure is preserved across kexec and allows the new kernel to
> + * identify which kernel it was booted from and how many kexec reboots
> + * have occurred.
> + */
> +struct kho_kexec_metadata {
> +	char previous_release[__NEW_UTS_LEN + 1];
> +	u32 kexec_count;
> +} __packed;
> +
[...]

-- 
Regards,
Pratyush Yadav