On Mon, Jan 26, 2026 at 08:07:25AM -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 <[email protected]>
> Acked-by: SeongJae Park <[email protected]>
Reviewed-by: Mike Rapoport (Microsoft) <[email protected]>
with a small nit below
> ---
> include/linux/kho/abi/kexec_handover.h | 31 ++++++++++++++
> kernel/liveupdate/kexec_handover.c | 74
> ++++++++++++++++++++++++++++++++++
> 2 files changed, 105 insertions(+)
>
> diff --git a/include/linux/kho/abi/kexec_handover.h
> b/include/linux/kho/abi/kexec_handover.h
> index 2201a0d2c159a..1f017756e9991 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 +85,36 @@
> /* 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
> + * @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.
> + *
> + * __NEW_UTS_LEN is part of UAPI, so it safe to use it in here.
Nit: it's usually referred to as uABI.
> + */
> +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
> *
--
Sincerely yours,
Mike.
