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

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Message-Id: <20260102-kho-v2-2-1747b1a3a1d6@debian.org>
Date: Fri, 02 Jan 2026 06:53:24 -0800
From: Breno Leitao <leitao@...ian.org>
To: 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, usamaarif642@...il.com, rmikey@...a.com, clm@...com, 
 riel@...riel.com, Breno Leitao <leitao@...ian.org>, kernel-team@...a.com
Subject: [PATCH v2 2/2] kexec: history: track kexec boot counter

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);
+	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) { }

-- 
2.47.3