kexec: add Kexec HandOver (KHO) generation helpers

Add the infrastructure to generate Kexec HandOver metadata.  Kexec
HandOver is a mechanism that allows Linux to preserve state - arbitrary
properties as well as memory locations - across kexec.

It does so using 2 concepts:

  1) KHO FDT - Every KHO kexec carries a KHO specific flattened device tree
     blob that describes preserved memory regions. Device drivers can
     register to KHO to serialize and preserve their states before kexec.

  2) Scratch Regions - CMA regions that we allocate in the first kernel.
     CMA gives us the guarantee that no handover pages land in those
     regions, because handover pages must be at a static physical memory
     location. We use these regions as the place to load future kexec
     images so that they won't collide with any handover data.

Link: https://lkml.kernel.org/r/20250509074635.3187114-5-changyuanl@google.com
Signed-off-by: Alexander Graf <graf@amazon.com>
Co-developed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Co-developed-by: Pratyush Yadav <ptyadav@amazon.de>
Signed-off-by: Pratyush Yadav <ptyadav@amazon.de>
Co-developed-by: Changyuan Lyu <changyuanl@google.com>
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: 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>

MAINTAINERS

@@ -13139,6 +13139,15 @@ F:	include/linux/kexec.h
 F:	include/uapi/linux/kexec.h
 F:	kernel/kexec*
 
+KEXEC HANDOVER (KHO)
+M:	Alexander Graf <graf@amazon.com>
+M:	Mike Rapoport <rppt@kernel.org>
+M:	Changyuan Lyu <changyuanl@google.com>
+L:	kexec@lists.infradead.org
+S:	Maintained
+F:	include/linux/kexec_handover.h
+F:	kernel/kexec_handover.c
+
 KEYS-ENCRYPTED
 M:	Mimi Zohar <zohar@linux.ibm.com>
 L:	linux-integrity@vger.kernel.org

include/linux/kexec_handover.h

@@ -0,0 +1,59 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef LINUX_KEXEC_HANDOVER_H
+#define LINUX_KEXEC_HANDOVER_H
+
+#include <linux/types.h>
+#include <linux/errno.h>
+
+struct kho_scratch {
+	phys_addr_t addr;
+	phys_addr_t size;
+};
+
+/* KHO Notifier index */
+enum kho_event {
+	KEXEC_KHO_FINALIZE = 0,
+	KEXEC_KHO_ABORT = 1,
+};
+
+struct notifier_block;
+
+struct kho_serialization;
+
+#ifdef CONFIG_KEXEC_HANDOVER
+bool kho_is_enabled(void);
+
+int kho_add_subtree(struct kho_serialization *ser, const char *name, void *fdt);
+
+int register_kho_notifier(struct notifier_block *nb);
+int unregister_kho_notifier(struct notifier_block *nb);
+
+void kho_memory_init(void);
+#else
+static inline bool kho_is_enabled(void)
+{
+	return false;
+}
+
+static inline int kho_add_subtree(struct kho_serialization *ser,
+				  const char *name, void *fdt)
+{
+	return -EOPNOTSUPP;
+}
+
+static inline int register_kho_notifier(struct notifier_block *nb)
+{
+	return -EOPNOTSUPP;
+}
+
+static inline int unregister_kho_notifier(struct notifier_block *nb)
+{
+	return -EOPNOTSUPP;
+}
+
+static inline void kho_memory_init(void)
+{
+}
+#endif /* CONFIG_KEXEC_HANDOVER */
+
+#endif /* LINUX_KEXEC_HANDOVER_H */

kernel/Makefile

@@ -80,6 +80,7 @@ obj-$(CONFIG_CRASH_DUMP) += crash_core.o
 obj-$(CONFIG_KEXEC) += kexec.o
 obj-$(CONFIG_KEXEC_FILE) += kexec_file.o
 obj-$(CONFIG_KEXEC_ELF) += kexec_elf.o
+obj-$(CONFIG_KEXEC_HANDOVER) += kexec_handover.o
 obj-$(CONFIG_BACKTRACE_SELF_TEST) += backtracetest.o
 obj-$(CONFIG_COMPAT) += compat.o
 obj-$(CONFIG_CGROUPS) += cgroup/

mm/mm_init.c

@@ -30,6 +30,7 @@
 #include <linux/crash_dump.h>
 #include <linux/execmem.h>
 #include <linux/vmstat.h>
+#include <linux/kexec_handover.h>
 #include <linux/hugetlb.h>
 #include "internal.h"
 #include "slab.h"
@@ -2770,6 +2771,13 @@ void __init mm_core_init(void)
 	report_meminit();
 	kmsan_init_shadow();
 	stack_depot_early_init();
+
+	/*
+	 * KHO memory setup must happen while memblock is still active, but
+	 * as close as possible to buddy initialization
+	 */
+	kho_memory_init();
+
 	memblock_free_all();
 	mem_init();
 	kmem_cache_init();