kho: persist blob size in KHO FDT

kho_add_subtree() accepts a size parameter but only forwards it to
debugfs.  The size is not persisted in the KHO FDT, so it is lost across
kexec.  This makes it impossible for the incoming kernel to determine the
blob size without understanding the blob format.

Store the blob size as a "blob-size" property in the KHO FDT alongside the
"preserved-data" physical address.  This allows the receiving kernel to
recover the size for any blob regardless of format.

Also extend kho_retrieve_subtree() with an optional size output parameter
so callers can learn the blob size without needing to understand the blob
format.  Update all callers to pass NULL for the new parameter.

Link: https://lore.kernel.org/20260316-kho-v9-3-ed6dcd951988@debian.org
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
Cc: Alexander Graf <graf@amazon.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

2 files changed, 19 insertions, 6 deletions

diff --git a/include/linux/kexec_handover.h b/include/linux/kexec_handover.h
index 0666cf298c7f..8968c56d2d73 100644
--- a/include/linux/kexec_handover.h
+++ b/include/linux/kexec_handover.h
@@ -34,7 +34,7 @@ struct page *kho_restore_pages(phys_addr_t phys, unsigned long nr_pages);
 void *kho_restore_vmalloc(const struct kho_vmalloc *preservation);
 int kho_add_subtree(const char *name, void *blob, size_t size);
 void kho_remove_subtree(void *blob);
-int kho_retrieve_subtree(const char *name, phys_addr_t *phys);
+int kho_retrieve_subtree(const char *name, phys_addr_t *phys, size_t *size);
 
 void kho_memory_init(void);
 
@@ -104,7 +104,8 @@ static inline int kho_add_subtree(const char *name, void *blob, size_t size)
 
 static inline void kho_remove_subtree(void *blob) { }
 
-static inline int kho_retrieve_subtree(const char *name, phys_addr_t *phys)
+static inline int kho_retrieve_subtree(const char *name, phys_addr_t *phys,
+				       size_t *size)
 {
 	return -EOPNOTSUPP;
 }
diff --git a/include/linux/kho/abi/kexec_handover.h b/include/linux/kho/abi/kexec_handover.h
index 6b7d8ef550f9..7e847a2339b0 100644
--- a/include/linux/kho/abi/kexec_handover.h
+++ b/include/linux/kho/abi/kexec_handover.h
@@ -41,25 +41,28 @@
  *   restore the preserved data.::
  *
  *     / {
- *         compatible = "kho-v2";
+ *         compatible = "kho-v3";
  *
  *         preserved-memory-map = <0x...>;
  *
  *         <subnode-name-1> {
  *             preserved-data = <0x...>;
+ *             blob-size = <0x...>;
  *         };
  *
  *         <subnode-name-2> {
  *             preserved-data = <0x...>;
+ *             blob-size = <0x...>;
  *         };
  *               ... ...
  *         <subnode-name-N> {
  *             preserved-data = <0x...>;
+ *             blob-size = <0x...>;
  *         };
  *     };
  *
  *   Root KHO Node (/):
- *     - compatible: "kho-v2"
+ *     - compatible: "kho-v3"
  *
  *       Indentifies the overall KHO ABI version.
  *
@@ -78,16 +81,25 @@
  *
  *       Physical address pointing to a subnode data blob that is also
  *       being preserved.
+ *
+ *     - blob-size: u64
+ *
+ *       Size in bytes of the preserved data blob. This is needed because
+ *       blobs may use arbitrary formats (not just FDT), so the size
+ *       cannot be determined from the blob content alone.
  */
 
 /* The compatible string for the KHO FDT root node. */
-#define KHO_FDT_COMPATIBLE "kho-v2"
+#define KHO_FDT_COMPATIBLE "kho-v3"
 
 /* The FDT property for the preserved memory map. */
 #define KHO_FDT_MEMORY_MAP_PROP_NAME "preserved-memory-map"
 
 /* The FDT property for preserved data blobs. */
-#define KHO_FDT_SUB_TREE_PROP_NAME "preserved-data"
+#define KHO_SUB_TREE_PROP_NAME "preserved-data"
+
+/* The FDT property for the size of preserved data blobs. */
+#define KHO_SUB_TREE_SIZE_PROP_NAME "blob-size"
 
 /**
  * DOC: Kexec Handover ABI for vmalloc Preservation