kho: remove finalize state and clients

Eliminate the `kho_finalize()` function and its associated state from the
KHO subsystem.  The transition to a radix tree for memory tracking makes
the explicit "finalize" state and its serialization step obsolete.

Remove the `kho_finalize()` and `kho_finalized()` APIs and their stub
implementations.  Update KHO client code and the debugfs interface to no
longer call or depend on the `kho_finalize()` mechanism.

Complete the move towards a stateless KHO, simplifying the overall design
by removing unnecessary state management.

Link: https://lkml.kernel.org/r/20260206021428.3386442-3-jasonmiu@google.com
Signed-off-by: Jason Miu <jasonmiu@google.com>
Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Alexander Graf <graf@amazon.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Changyuan Lyu <changyuanl@google.com>
Cc: David Matlack <dmatlack@google.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Pratyush Yadav <pratyush@kernel.org>
Cc: Ran Xiaokai <ran.xiaokai@zte.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/admin-guide/mm/kho.rst

@@ -28,20 +28,10 @@ per NUMA node scratch regions on boot.
 Perform a KHO kexec
 ===================
 
-First, before you perform a KHO kexec, you need to move the system into
-the :ref:`KHO finalization phase <kho-finalization-phase>` ::
-
-  $ echo 1 > /sys/kernel/debug/kho/out/finalize
-
-After this command, the KHO FDT is available in
-``/sys/kernel/debug/kho/out/fdt``. Other subsystems may also register
-their own preserved sub FDTs under
-``/sys/kernel/debug/kho/out/sub_fdts/``.
-
-Next, load the target payload and kexec into it. It is important that you
-use the ``-s`` parameter to use the in-kernel kexec file loader, as user
-space kexec tooling currently has no support for KHO with the user space
-based file loader ::
+To perform a KHO kexec, load the target payload and kexec into it. It
+is important that you use the ``-s`` parameter to use the in-kernel
+kexec file loader, as user space kexec tooling currently has no
+support for KHO with the user space based file loader ::
 
   # kexec -l /path/to/bzImage --initrd /path/to/initrd -s
   # kexec -e
@@ -52,40 +42,19 @@ For example, if you used ``reserve_mem`` command line parameter to create
 an early memory reservation, the new kernel will have that memory at the
 same physical address as the old kernel.
 
-Abort a KHO exec
-================
-
-You can move the system out of KHO finalization phase again by calling ::
-
-  $ echo 0 > /sys/kernel/debug/kho/out/active
-
-After this command, the KHO FDT is no longer available in
-``/sys/kernel/debug/kho/out/fdt``.
-
 debugfs Interfaces
 ==================
 
+These debugfs interfaces are available when the kernel is compiled with
+``CONFIG_KEXEC_HANDOVER_DEBUGFS`` enabled.
+
 Currently KHO creates the following debugfs interfaces. Notice that these
 interfaces may change in the future. They will be moved to sysfs once KHO is
 stabilized.
 
-``/sys/kernel/debug/kho/out/finalize``
-    Kexec HandOver (KHO) allows Linux to transition the state of
-    compatible drivers into the next kexec'ed kernel. To do so,
-    device drivers will instruct KHO to preserve memory regions,
-    which could contain serialized kernel state.
-    While the state is serialized, they are unable to perform
-    any modifications to state that was serialized, such as
-    handed over memory allocations.
-
-    When this file contains "1", the system is in the transition
-    state. When contains "0", it is not. To switch between the
-    two states, echo the respective number into this file.
-
 ``/sys/kernel/debug/kho/out/fdt``
-    When KHO state tree is finalized, the kernel exposes the
-    flattened device tree blob that carries its current KHO
-    state in this file. Kexec user space tooling can use this
+    The kernel exposes the flattened device tree blob that carries its
+    current KHO state in this file. Kexec user space tooling can use this
     as input file for the KHO payload image.
 
 ``/sys/kernel/debug/kho/out/scratch_len``
@@ -100,8 +69,8 @@ stabilized.
     it should place its payload images.
 
 ``/sys/kernel/debug/kho/out/sub_fdts/``
-    In the KHO finalization phase, KHO producers register their own
-    FDT blob under this directory.
+    KHO producers can register their own FDT or another binary blob under
+    this directory.
 
 ``/sys/kernel/debug/kho/in/fdt``
     When the kernel was booted with Kexec HandOver (KHO),

Documentation/core-api/kho/index.rst

@@ -71,18 +71,6 @@ for boot memory allocations and as target memory for kexec blobs, some parts
 of that memory region may be reserved. These reservations are irrelevant for
 the next KHO, because kexec can overwrite even the original kernel.
 
-.. _kho-finalization-phase:
-
-KHO finalization phase
-======================
-
-To enable user space based kexec file loader, the kernel needs to be able to
-provide the FDT that describes the current kernel's state before
-performing the actual kexec. The process of generating that FDT is
-called serialization. When the FDT is generated, some properties
-of the system may become immutable because they are already written down
-in the FDT. That state is called the KHO finalization phase.
-
 Kexec Handover Radix Tree
 =========================
 

kernel/liveupdate/kexec_handover.c

@@ -68,8 +68,7 @@ early_param("kho", kho_parse_enable);
 
 struct kho_out {
 	void *fdt;
-	bool finalized;
-	struct mutex lock; /* protects KHO FDT finalization */
+	struct mutex lock; /* protects KHO FDT */
 
 	struct kho_radix_tree radix_tree;
 	struct kho_debugfs dbg;
@@ -80,7 +79,6 @@ static struct kho_out kho_out = {
 	.radix_tree = {
 		.lock = __MUTEX_INITIALIZER(kho_out.radix_tree.lock),
 	},
-	.finalized = false,
 };
 
 /**
@@ -1241,23 +1239,6 @@ void kho_restore_free(void *mem)
 }
 EXPORT_SYMBOL_GPL(kho_restore_free);
 
-int kho_finalize(void)
-{
-	if (!kho_enable)
-		return -EOPNOTSUPP;
-
-	guard(mutex)(&kho_out.lock);
-	kho_out.finalized = true;
-
-	return 0;
-}
-
-bool kho_finalized(void)
-{
-	guard(mutex)(&kho_out.lock);
-	return kho_out.finalized;
-}
-
 struct kho_in {
 	phys_addr_t fdt_phys;
 	phys_addr_t scratch_phys;

kernel/liveupdate/kexec_handover_debugfs.c

@@ -76,24 +76,6 @@ void kho_debugfs_fdt_remove(struct kho_debugfs *dbg, void *fdt)
 	}
 }
 
-static int kho_out_finalize_get(void *data, u64 *val)
-{
-	*val = kho_finalized();
-
-	return 0;
-}
-
-static int kho_out_finalize_set(void *data, u64 val)
-{
-	if (val)
-		return kho_finalize();
-	else
-		return -EINVAL;
-}
-
-DEFINE_DEBUGFS_ATTRIBUTE(kho_out_finalize_fops, kho_out_finalize_get,
-			 kho_out_finalize_set, "%llu\n");
-
 static int scratch_phys_show(struct seq_file *m, void *v)
 {
 	for (int i = 0; i < kho_scratch_cnt; i++)
@@ -199,11 +181,6 @@ __init int kho_out_debugfs_init(struct kho_debugfs *dbg)
 	if (IS_ERR(f))
 		goto err_rmdir;
 
-	f = debugfs_create_file("finalize", 0600, dir, NULL,
-				&kho_out_finalize_fops);
-	if (IS_ERR(f))
-		goto err_rmdir;
-
 	dbg->dir = dir;
 	dbg->sub_fdt_dir = sub_fdt_dir;
 	return 0;

kernel/liveupdate/kexec_handover_internal.h

@@ -22,9 +22,6 @@ struct kho_debugfs {};
 extern struct kho_scratch *kho_scratch;
 extern unsigned int kho_scratch_cnt;
 
-bool kho_finalized(void);
-int kho_finalize(void);
-
 #ifdef CONFIG_KEXEC_HANDOVER_DEBUGFS
 int kho_debugfs_init(void);
 void kho_in_debugfs_init(struct kho_debugfs *dbg, const void *fdt);

kernel/liveupdate/luo_core.c

@@ -230,17 +230,7 @@ int liveupdate_reboot(void)
 
 	luo_flb_serialize();
 
-	err = kho_finalize();
-	if (err) {
-		pr_err("kho_finalize failed %d\n", err);
-		/*
-		 * kho_finalize() may return libfdt errors, to aboid passing to
-		 * userspace unknown errors, change this to EAGAIN.
-		 */
-		err = -EAGAIN;
-	}
-
-	return err;
+	return 0;
 }
 
 /**

tools/testing/selftests/kho/init.c

@@ -11,7 +11,6 @@
 /* from arch/x86/include/asm/setup.h */
 #define COMMAND_LINE_SIZE	2048
 
-#define KHO_FINALIZE "/debugfs/kho/out/finalize"
 #define KERNEL_IMAGE "/kernel"
 
 static int mount_filesystems(void)
@@ -22,22 +21,6 @@ static int mount_filesystems(void)
 	return mount("proc", "/proc", "proc", 0, NULL);
 }
 
-static int kho_enable(void)
-{
-	const char enable[] = "1";
-	int fd;
-
-	fd = open(KHO_FINALIZE, O_RDWR);
-	if (fd < 0)
-		return -1;
-
-	if (write(fd, enable, sizeof(enable)) != sizeof(enable))
-		return 1;
-
-	close(fd);
-	return 0;
-}
-
 static long kexec_file_load(int kernel_fd, int initrd_fd,
 			    unsigned long cmdline_len, const char *cmdline,
 			    unsigned long flags)
@@ -78,9 +61,6 @@ int main(int argc, char *argv[])
 	if (mount_filesystems())
 		goto err_reboot;
 
-	if (kho_enable())
-		goto err_reboot;
-
 	if (kexec_load())
 		goto err_reboot;