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

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Message-ID: <fc542a5e-db54-4d7e-b363-42cc253b1a8c@gmail.com>
Date: Fri, 2 Jan 2026 18:09:57 +0300
From: Usama Arif <usamaarif642@...il.com>
To: Breno Leitao <leitao@...ian.org>, Alexander Graf <graf@...zon.com>,
 Mike Rapoport <rppt@...nel.org>, Pasha Tatashin <pasha.tatashin@...een.com>,
 Pratyush Yadav <pratyush@...nel.org>
Cc: linux-kernel@...r.kernel.org, kexec@...ts.infradead.org,
 linux-mm@...ck.org, 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 02/01/2026 17:53, Breno Leitao 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);

Should this be kexec_count = cpu_to_fdt32(kho_in.kexec_count) + 1; ?

> +	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))
> +		/*
> +		 * 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) { }
>