[RFC v1 6/9] kho: introduce compatibility helpers and decouple block version

[Pasha Tatashin]

Decouple the block compatibility string from the global KHO version.

Introduce a compatibility helper header (compat.h) defining utility
macros for constructing subsystem compatibility strings, specifically:
- KHO_SUB_COMPAT() to append sub-component compatibility strings using
  a semicolon separator.
- KHO_COMPAT_ALIGN() to align compatibility string sizes to 8-byte
  boundaries.

Define the individual block compatibility string "block-v1" in block.h,
and integrate it into the composite LUO compatibility string (LUO_ABI_COMPATIBLE)
via the new compatibility helpers.

Signed-off-by: Pasha Tatashin <pasha.tatashin at soleen.com>
---
 include/linux/kho/abi/block.h  |  4 +++-
 include/linux/kho/abi/compat.h | 33 +++++++++++++++++++++++++++++++++
 include/linux/kho/abi/luo.h    |  8 ++++++--
 3 files changed, 42 insertions(+), 3 deletions(-)
 create mode 100644 include/linux/kho/abi/compat.h

diff --git a/include/linux/kho/abi/block.h b/include/linux/kho/abi/block.h
index d06d64b963be..95d13cf677cf 100644
--- a/include/linux/kho/abi/block.h
+++ b/include/linux/kho/abi/block.h
@@ -14,7 +14,7 @@
  * This interface is a contract. Any modification to the structure fields,
  * compatible strings, or the layout of the `__packed` serialization
  * structures defined here constitutes a breaking change. Such changes require
- * incrementing the version number in the `KHO_FDT_COMPATIBLE` string to
+ * incrementing the version number in the `KHO_BLOCK_COMPATIBLE` string to
  * prevent a new kernel from misinterpreting data from an old kernel.
  *
  * Changes are allowed provided the compatibility version is incremented;
@@ -28,6 +28,8 @@
 #include <asm/page.h>
 #include <linux/types.h>
 
+#define KHO_BLOCK_COMPATIBLE "block-v1"
+
 /**
  * KHO_BLOCK_SIZE - The size of each serialization block.
  *
diff --git a/include/linux/kho/abi/compat.h b/include/linux/kho/abi/compat.h
new file mode 100644
index 000000000000..25edd964c390
--- /dev/null
+++ b/include/linux/kho/abi/compat.h
@@ -0,0 +1,33 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (c) 2026 Google LLC.
+ * Pasha Tatashin <pasha.tatashin at soleen.com>
+ */
+
+#ifndef _LINUX_KHO_ABI_COMPAT_H
+#define _LINUX_KHO_ABI_COMPAT_H
+
+#include <linux/align.h>
+
+/**
+ * KHO_SUB_COMPAT - Helper to append a sub-component compatibility string.
+ * @str: The compatibility string of the sub-component.
+ *
+ * Appends a KHO safe data structure compatibility string to a sub-system
+ * compatibility string using a semicolon ';' as a separator.
+ *
+ * NOTE: Sub-components MUST be added in strict alphabetical order to maintain
+ * a consistent and predictable compatibility string value.
+ */
+#define KHO_SUB_COMPAT(str) ";" str
+
+/**
+ * KHO_COMPAT_ALIGN - Align a compatibility string size to 8 bytes.
+ * @str: The compatibility string.
+ *
+ * Aligns the size of a compatibility string to an 8-byte boundary for use
+ * in ABI structures.
+ */
+#define KHO_COMPAT_ALIGN(str)	ALIGN(sizeof(str), 8)
+
+#endif /* _LINUX_KHO_ABI_COMPAT_H */
diff --git a/include/linux/kho/abi/luo.h b/include/linux/kho/abi/luo.h
index 288076de6d4a..b502670cd2a6 100644
--- a/include/linux/kho/abi/luo.h
+++ b/include/linux/kho/abi/luo.h
@@ -58,6 +58,7 @@
 #define _LINUX_KHO_ABI_LUO_H
 
 #include <linux/align.h>
+#include <linux/kho/abi/compat.h>
 #include <linux/kho/abi/block.h>
 #include <uapi/linux/liveupdate.h>
 
@@ -65,8 +66,11 @@
  * The LUO state is registered under this KHO entry name.
  */
 #define LUO_KHO_ENTRY_NAME	"LUO"
-#define LUO_ABI_COMPATIBLE	"luo-v5"
-#define LUO_ABI_COMPAT_LEN	ALIGN(sizeof(LUO_ABI_COMPATIBLE), 8)
+#define LUO_ABI_COMPAT_BASE	"luo-v5"
+#define LUO_ABI_COMPATIBLE						\
+	LUO_ABI_COMPAT_BASE						\
+	KHO_SUB_COMPAT(KHO_BLOCK_COMPATIBLE)
+#define LUO_ABI_COMPAT_LEN	KHO_COMPAT_ALIGN(LUO_ABI_COMPATIBLE)
 
 /**
  * struct luo_ser - Centralized LUO ABI header.
-- 
2.53.0