linux-kernel - Re: [PATCH v2 2/2] kexec: history: track kexec boot counter

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Message-ID: <CA+CK2bCnBD6afWcbdmqPFBmybeH8BKLo8YSbSCUEJEY+hOij4w@mail.gmail.com>
Date: Fri, 2 Jan 2026 11:20:49 -0500
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Breno Leitao <leitao@...ian.org>
Cc: Alexander Graf <graf@...zon.com>, Mike Rapoport <rppt@...nel.org>, 
	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
Subject: Re: [PATCH v2 2/2] kexec: history: track kexec boot counter

On Fri, Jan 2, 2026 at 9:53 AM Breno Leitao <leitao@...ian.org> wrote:
>
> Track and display the number of kexec boots since the last cold reboot
> when CONFIG_KEXEC_HISTORY is enabled.
>
> This extends the previous kernel release tracking feature by adding
> a counter that increments with each kexec boot. The counter provides
> visibility into the kexec chain depth, which is useful for understanding
> boot history in production environments.
>
> Add a new property, "kexec-count" in KHO FDT alongside the existing
> "previous-release" property. The counter is:
>
>  - Initialized to 0 when kho_in is instantiated.
>  - Incremented by 1 on each subsequent kexec.
>  - Printed alongside the previous kernel release version.
>
> The counter is stored as a 32-bit unsigned integer in FDT format and is
> only active when CONFIG_KEXEC_HISTORY is enabled.
>
> Signed-off-by: Breno Leitao <leitao@...ian.org>
> Suggested-by: Pasha Tatashin <pasha.tatashin@...een.com>
> ---
>  kernel/liveupdate/kexec_handover.c | 25 +++++++++++++++++++++++--
>  1 file changed, 23 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> index 06d99627bb3c..fe5a2c5c4c86 100644
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c
> @@ -38,6 +38,7 @@
>  #define PROP_PRESERVED_MEMORY_MAP "preserved-memory-map"
>  #define PROP_SUB_FDT "fdt"
>  #define PROP_PREVIOUS_RELEASE "previous-release"
> +#define PROP_KEXEC_COUNT "kexec-count"
>
>  #define KHO_PAGE_MAGIC 0x4b484f50U /* ASCII for 'KHOP' */
>
> @@ -1257,6 +1258,7 @@ struct kho_in {
>         phys_addr_t scratch_phys;
>  #ifdef CONFIG_KEXEC_HISTORY
>         char previous_release[__NEW_UTS_LEN + 1];
> +       u32 kexec_count;
>  #endif
>         struct kho_debugfs dbg;
>  };
> @@ -1330,6 +1332,9 @@ static __init int kho_out_fdt_setup(void)
>         void *root = kho_out.fdt;
>         u64 empty_mem_map = 0;
>         int err;
> +#ifdef CONFIG_KEXEC_HISTORY
> +       u32 kexec_count;
> +#endif
>
>         err = fdt_create(root, PAGE_SIZE);
>         err |= fdt_finish_reservemap(root);
> @@ -1340,6 +1345,10 @@ static __init int kho_out_fdt_setup(void)
>  #ifdef CONFIG_KEXEC_HISTORY
>         err |= fdt_property_string(root, PROP_PREVIOUS_RELEASE,
>                                    init_uts_ns.name.release);
> +       /* kho_in.kexec_count is set to 0 on cold boot */
> +       kexec_count = cpu_to_fdt32(kho_in.kexec_count + 1);

For KHO preserved data we use native endiness, so do not bother with
cpu_to_fdt32.

> +       err |= fdt_property(root, PROP_KEXEC_COUNT, &kexec_count,
> +                           sizeof(kexec_count));
>  #endif
>         err |= fdt_end_node(root);
>         err |= fdt_finish(root);
> @@ -1468,6 +1477,7 @@ void __init kho_memory_init(void)
>  static void __init kho_print_previous_kernel(const void *fdt)
>  {
>         const char *prev_release;
> +       const u32 *count_ptr;
>         int len;
>
>         prev_release = fdt_getprop(fdt, 0, PROP_PREVIOUS_RELEASE, &len);
> @@ -1476,8 +1486,19 @@ static void __init kho_print_previous_kernel(const void *fdt)
>
>         strscpy(kho_in.previous_release, prev_release,
>                 sizeof(kho_in.previous_release));
> -       pr_info("This kernel was kexec'ed from kernel release: %s\n",
> -               kho_in.previous_release);
> +
> +       /* Read the kexec count from the previous kernel */
> +       count_ptr = fdt_getprop(fdt, 0, PROP_KEXEC_COUNT, &len);
> +       if (WARN_ON_ONCE(!count_ptr || len <= 0))

This function is called only once anway.

> +               /*
> +                * PROP_KEXEC_COUNT should exist if PROP_PREVIOUS_RELEASE
> +                * exists.
> +                */
> +               return;
> +       kho_in.kexec_count = fdt32_to_cpu(*count_ptr);
> +
> +       pr_info("This kernel was kexec'ed from kernel release: %s (kexec count: %u)\n",
> +               kho_in.previous_release, kho_in.kexec_count);
>  }
>  #else
>  static void __init kho_print_previous_kernel(const void *fdt) { }
>
> --
> 2.47.3
>