linux-kernel - [PATCH v3 2/2] kho: history: track kexec boot counter

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Message-Id: <20260108-kho-v3-2-b1d6b7a89342@debian.org>
Date: Thu, 08 Jan 2026 08:40:59 -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 v3 2/2] kho: history: track kexec boot counter

Track and display the number of kexec boots since the last cold reboot.

This extends the previous kernel release tracking 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 on cold boot (when kho_in is first 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.

This is different than counter for LUO, given KHO can be used
independently from LUO.

Also WARN() if KHO_PROP_PREVIOUS_RELEASE doesn't exist, because it
must exist if KHO_PROP_PREVIOUS_RELEASE exists.

Signed-off-by: Breno Leitao <leitao@...ian.org>
Suggested-by: Pasha Tatashin <pasha.tatashin@...een.com>
---
 include/linux/kho/abi/kexec_handover.h |  3 +++
 kernel/liveupdate/kexec_handover.c     | 16 +++++++++++++++-
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/include/linux/kho/abi/kexec_handover.h b/include/linux/kho/abi/kexec_handover.h
index f4f31e8f575b..9c4fc4c6a212 100644
--- a/include/linux/kho/abi/kexec_handover.h
+++ b/include/linux/kho/abi/kexec_handover.h
@@ -87,6 +87,9 @@
 /* The FDT property to track previous kernel (kexec caller) */
 #define KHO_PROP_PREVIOUS_RELEASE "previous-release"
 
+/* The FDT property to track number of kexec counts so far */
+#define KHO_PROP_KEXEC_COUNT "kexec-count"
+
 /**
  * DOC: Kexec Handover ABI for vmalloc Preservation
  *
diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
index b2d57868d22f..34cfc98076b8 100644
--- a/kernel/liveupdate/kexec_handover.c
+++ b/kernel/liveupdate/kexec_handover.c
@@ -1248,6 +1248,7 @@ struct kho_in {
 	phys_addr_t scratch_phys;
 	phys_addr_t mem_map_phys;
 	char previous_release[__NEW_UTS_LEN + 1];
+	u32 kexec_count;
 	struct kho_debugfs dbg;
 };
 
@@ -1319,6 +1320,7 @@ static __init int kho_out_fdt_setup(void)
 {
 	void *root = kho_out.fdt;
 	u64 empty_mem_map = 0;
+	u32 kexec_count;
 	int err;
 
 	err = fdt_create(root, PAGE_SIZE);
@@ -1329,6 +1331,10 @@ static __init int kho_out_fdt_setup(void)
 			    sizeof(empty_mem_map));
 	err |= fdt_property_string(root, KHO_PROP_PREVIOUS_RELEASE,
 				   init_uts_ns.name.release);
+	/* kho_in.kexec_count is set to 0 on cold boot */
+	kexec_count = kho_in.kexec_count + 1;
+	err |= fdt_property(root, KHO_PROP_KEXEC_COUNT, &kexec_count,
+			    sizeof(kexec_count));
 	err |= fdt_end_node(root);
 	err |= fdt_finish(root);
 
@@ -1443,15 +1449,23 @@ void __init kho_memory_init(void)
 static int __init kho_print_previous_kernel(const void *fdt)
 {
 	const char *prev_release;
+	const u32 *count_ptr;
 	int len;
 
 	prev_release = fdt_getprop(fdt, 0, KHO_PROP_PREVIOUS_RELEASE, &len);
 	if (!prev_release || len <= 0)
 		return -ENOENT;
 
+	/* Read the kexec count from the previous kernel */
+	count_ptr = fdt_getprop(fdt, 0, KHO_PROP_KEXEC_COUNT, &len);
+	if (WARN_ON(!count_ptr || len != sizeof(u32)))
+		return -ENOENT;
+	kho_in.kexec_count = *count_ptr;
+
 	strscpy(kho_in.previous_release, prev_release,
 		sizeof(kho_in.previous_release));
-	pr_info("exec from: %s\n", kho_in.previous_release);
+	pr_info("exec from: %s (count %u)\n", kho_in.previous_release,
+					      kho_in.kexec_count);
 
 	return 0;
 }

-- 
2.47.3