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Message-ID: <aXICriRKbYB5f3li@kernel.org>
Date: Thu, 22 Jan 2026 12:57:50 +0200
From: Mike Rapoport <rppt@...nel.org>
To: Breno Leitao <leitao@...ian.org>
Cc: Alexander Graf <graf@...zon.com>,
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 at 06:50:38AM -0800, 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>
> ---
> Changes in v4:
> - Squashed everything in a single commit
> - Moved from FDT to C structs (Pratyush)
> - Usage of subtress intead of FDT directly (Pratyush)
> - Renamed a bunch of variables and functions.
> - Link to v3: https://patch.msgid.link/20260108-kho-v3-0-b1d6b7a89342@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
>
> Changes from v2 to v1 (RFC)
> - 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
> ---
> include/linux/kho/abi/kexec_handover.h | 29 +++++++++++++++
> kernel/liveupdate/kexec_handover.c | 65 ++++++++++++++++++++++++++++++++++
> 2 files changed, 94 insertions(+)
>
> 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
> + *
It would be nice to link it from Documentation/ as well ;-)
> + * 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
> + * @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;
> +
> +#define KHO_METADATA_NODE_NAME "kexec-metadata"
> +
> /**
> * DOC: Kexec Handover ABI for vmalloc Preservation
> *
> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
...
> static __init int kho_init(void)
> {
> const void *fdt = kho_get_fdt();
> @@ -1357,6 +1413,15 @@ static __init int kho_init(void)
> if (err)
> goto err_free_fdt;
>
> + if (fdt)
> + kho_process_kexec_metadata();
Can't we move it into the existing if (fdt) below?
> +
> + /* Populate kexec metadata for the possible next kexec */
> + err = kho_populate_kexec_metadata();
> + if (err)
> + pr_warn("failed to initialize kexec-metadata subtree: %d\n",
> + err);
Please follow if (err) goto err_ pattern.
kho_populate_kexec_metadata() failure essentially means that we failed to
allocate memory. This shouldn't happen that early in boot, but if it did,
then something is utterly wrong.
> +
> if (fdt) {
> kho_in_debugfs_init(&kho_in.dbg, fdt);
> return 0;
--
Sincerely yours,
Mike.
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