Documentation: KHO: add memblock bindings

We introduced KHO into Linux: A framework that allows Linux to pass
metadata and memory across kexec from Linux to Linux.  KHO reuses fdt as
file format and shares a lot of the same properties of firmware-to- Linux
boot formats: It needs a stable, documented ABI that allows for forward
and backward compatibility as well as versioning.

As first user of KHO, we introduced memblock which can now preserve memory
ranges reserved with reserve_mem command line options contents across
kexec, so you can use the post-kexec kernel to read traces from the
pre-kexec kernel.

This patch adds memblock schemas similar to "device" device tree ones to a
new kho bindings directory.  This allows us to force contributors to
document the data that moves across KHO kexecs and catch breaking change
during review.

Link: https://lkml.kernel.org/r/20250509074635.3187114-18-changyuanl@google.com
Co-developed-by: Alexander Graf <graf@amazon.com>
Signed-off-by: Alexander Graf <graf@amazon.com>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Changyuan Lyu <changyuanl@google.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Anthony Yznaga <anthony.yznaga@oracle.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Ashish Kalra <ashish.kalra@amd.com>
Cc: Ben Herrenschmidt <benh@kernel.crashing.org>
Cc: Borislav Betkov <bp@alien8.de>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Gowans <jgowans@amazon.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Krzysztof Kozlowski <krzk@kernel.org>
Cc: Marc Rutland <mark.rutland@arm.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Pratyush Yadav <ptyadav@amazon.de>
Cc: Rob Herring <robh@kernel.org>
Cc: Saravana Kannan <saravanak@google.com>
Cc: Stanislav Kinsburskii <skinsburskii@linux.microsoft.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleinxer <tglx@linutronix.de>
Cc: Thomas Lendacky <thomas.lendacky@amd.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/core-api/kho/bindings/memblock/memblock.yaml

@@ -0,0 +1,39 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+title: Memblock reserved memory
+
+maintainers:
+  - Mike Rapoport <rppt@kernel.org>
+
+description: |
+  Memblock can serialize its current memory reservations created with
+  reserve_mem command line option across kexec through KHO.
+  The post-KHO kernel can then consume these reservations and they are
+  guaranteed to have the same physical address.
+
+properties:
+  compatible:
+    enum:
+      - reserve-mem-v1
+
+patternProperties:
+  "$[0-9a-f_]+^":
+    $ref: reserve-mem.yaml#
+    description: reserved memory regions
+
+required:
+  - compatible
+
+additionalProperties: false
+
+examples:
+  - |
+    memblock {
+      compatible = "memblock-v1";
+      n1 {
+        compatible = "reserve-mem-v1";
+        start = <0xc06b 0x4000000>;
+        size = <0x04 0x00>;
+      };
+    };

Documentation/core-api/kho/bindings/memblock/reserve-mem.yaml

@@ -0,0 +1,40 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+title: Memblock reserved memory regions
+
+maintainers:
+  - Mike Rapoport <rppt@kernel.org>
+
+description: |
+  Memblock can serialize its current memory reservations created with
+  reserve_mem command line option across kexec through KHO.
+  This object describes each such region.
+
+properties:
+  compatible:
+    enum:
+      - reserve-mem-v1
+
+  start:
+    description: |
+      physical address (u64) of the reserved memory region.
+
+  size:
+    description: |
+      size (u64) of the reserved memory region.
+
+required:
+  - compatible
+  - start
+  - size
+
+additionalProperties: false
+
+examples:
+  - |
+    n1 {
+      compatible = "reserve-mem-v1";
+      start = <0xc06b 0x4000000>;
+      size = <0x04 0x00>;
+    };

MAINTAINERS

@@ -15448,6 +15448,7 @@ M:	Mike Rapoport <rppt@kernel.org>
 L:	linux-mm@kvack.org
 S:	Maintained
 F:	Documentation/core-api/boot-time-mm.rst
+F:	Documentation/core-api/kho/bindings/memblock/*
 F:	include/linux/memblock.h
 F:	mm/memblock.c
 F:	mm/mm_init.c