liveupdate: luo_core: Live Update Orchestrator

Patch series "Live Update Orchestrator", v8.

This series introduces the Live Update Orchestrator, a kernel subsystem
designed to facilitate live kernel updates using a kexec-based reboot.
This capability is critical for cloud environments, allowing hypervisors
to be updated with minimal downtime for running virtual machines. LUO
achieves this by preserving the state of selected resources, such as
memory, devices and their dependencies, across the kernel transition.

As a key feature, this series includes support for preserving memfd file
descriptors, which allows critical in-memory data, such as guest RAM or
any other large memory region, to be maintained in RAM across the kexec
reboot.

The other series that use LUO, are VFIO [1], IOMMU [2], and PCI [3]
preservations.

Github repo of this series [4].

The core of LUO is a framework for managing the lifecycle of preserved
resources through a userspace-driven interface. Key features include:

- Session Management
Userspace agent (i.e. luod [5]) creates named sessions, each
represented by a file descriptor (via centralized agent that controls
/dev/liveupdate). The lifecycle of all preserved resources within a
session is tied to this FD, ensuring automatic kernel cleanup if the
controlling userspace agent crashes or exits unexpectedly.

- File Preservation
A handler-based framework allows specific file types (demonstrated
here with memfd) to be preserved. Handlers manage the serialization,
restoration, and lifecycle of their specific file types.

- File-Lifecycle-Bound State
A new mechanism for managing shared global state whose lifecycle is
tied to the preservation of one or more files. This is crucial for
subsystems like IOMMU or HugeTLB, where multiple file descriptors may
depend on a single, shared underlying resource that must be preserved
only once.

- KHO Integration
LUO drives the Kexec Handover framework programmatically to pass its
serialized metadata to the next kernel. The LUO state is finalized and
added to the kexec image just before the reboot is triggered. In the
future this step will also be removed once stateless KHO is
merged [6].

- Userspace Interface
Control is provided via ioctl commands on /dev/liveupdate for creating
and retrieving sessions, as well as on session file descriptors for
managing individual files.

- Testing
The series includes a set of selftests, including userspace API
validation, kexec-based lifecycle tests for various session and file
scenarios, and a new in-kernel test module to validate the FLB logic.




Introduce LUO, a mechanism intended to facilitate kernel updates while
keeping designated devices operational across the transition (e.g., via
kexec). The primary use case is updating hypervisors with minimal
disruption to running virtual machines. For userspace side of hypervisor
update we have copyless migration. LUO is for updating the kernel.

This initial patch lays the groundwork for the LUO subsystem.

Further functionality, including the implementation of state transition
logic, integration with KHO, and hooks for subsystems and file
descriptors, will be added in subsequent patches.

Create a character device at /dev/liveupdate.

A new uAPI header, <uapi/linux/liveupdate.h>, will define the necessary
structures. The magic number for IOCTL is registered in
Documentation/userspace-api/ioctl/ioctl-number.rst.

Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lore.kernel.org/all/[e-mail]/ [1]
Link: https://lore.kernel.org/linux-iommu/[e-mail] [2]
Link: https://lore.kernel.org/linux-pci/[e-mail] [3]
Link: https://github.com/googleprodkernel/linux-liveupdate/tree/luo/v8 [4]
Link: https://tinyurl.com/luoddesign [5]
Link: https://lore.kernel.org/all/[e-mail] [6]
Link: https://lore.kernel.org/all/[e-mail] [7]
Link: https://github.com/soleen/linux/blob/luo/v8b03/diff.v7.v8 [8]
Signed-off-by: Pasha Tatashin <[e-mail]>
Reviewed-by: Pratyush Yadav <[e-mail]>
Reviewed-by: Mike Rapoport (Microsoft) <[e-mail]>
Tested-by: David Matlack <[e-mail]>
Cc: Aleksander Lobakin <[e-mail]>
Cc: Alexander Graf <[e-mail]>
Cc: Alice Ryhl <[e-mail]>
Cc: Andriy Shevchenko <[e-mail]>
Cc: anish kumar <[e-mail]>
Cc: Anna Schumaker <[e-mail]>
Cc: Bartosz Golaszewski <[e-mail]>
Cc: Bjorn Helgaas <[e-mail]>
Cc: Borislav Betkov <[e-mail]>
Cc: Chanwoo Choi <[e-mail]>
Cc: Chen Ridong <[e-mail]>
Cc: Chris Li <[e-mail]>
Cc: Christian Brauner <[e-mail]>
Cc: Daniel Wagner <[e-mail]>
Cc: Danilo Krummrich <[e-mail]>
Cc: Dan Williams <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: David Jeffery <[e-mail]>
Cc: David Rientjes <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Guixin Liu <[e-mail]>
Cc: "H. Peter Anvin" <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: Ilpo Järvinen <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Ira Weiny <[e-mail]>
Cc: Jann Horn <[e-mail]>
Cc: Jason Gunthorpe <[e-mail]>
Cc: Jens Axboe <[e-mail]>
Cc: Joanthan Cameron <[e-mail]>
Cc: Joel Granados <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: Lennart Poettering <[e-mail]>
Cc: Leon Romanovsky <[e-mail]>
Cc: Leon Romanovsky <[e-mail]>
Cc: Lukas Wunner <[e-mail]>
Cc: Marc Rutland <[e-mail]>
Cc: Masahiro Yamada <[e-mail]>
Cc: Matthew Maurer <[e-mail]>
Cc: Miguel Ojeda <[e-mail]>
Cc: Myugnjoo Ham <[e-mail]>
Cc: Parav Pandit <[e-mail]>
Cc: Randy Dunlap <[e-mail]>
Cc: Roman Gushchin <[e-mail]>
Cc: Saeed Mahameed <[e-mail]>
Cc: Samiullah Khawaja <[e-mail]>
Cc: Song Liu <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Cc: Stuart Hayes <[e-mail]>
Cc: Tejun Heo <[e-mail]>
Cc: Thomas Gleinxer <[e-mail]>
Cc: Thomas Weißschuh <[e-mail]>
Cc: Vincent Guittot <[e-mail]>
Cc: William Tu <[e-mail]>
Cc: Yoann Congal <[e-mail]>
Cc: Zijun Hu <[e-mail]>
Cc: Pratyush Yadav <[e-mail]>
Cc: Zhu Yanjun <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

Documentation/userspace-api/ioctl/ioctl-number.rst

···
 0xB8  01-02  uapi/misc/mrvl_cn10k_dpi.h                                Marvell CN10K DPI driver
 0xB8  all    uapi/linux/mshv.h                                         Microsoft Hyper-V /dev/mshv driver
                                                                        <mailto:[e-mail]>
+0xBA  00-0F  uapi/linux/liveupdate.h                                   Pasha Tatashin
+                                                                       <mailto:[e-mail]>
 0xC0  00-0F  linux/usb/iowarrior.h
 0xCA  00-0F  uapi/misc/cxl.h                                           Dead since 6.15
 0xCA  10-2F  uapi/misc/ocxl.h

include/linux/liveupdate.h

···
+/* SPDX-License-Identifier: GPL-2.0 */
+
+/*
+ * Copyright (c) 2025, Google LLC.
+ * Pasha Tatashin <[e-mail]>
+ */
+#ifndef _LINUX_LIVEUPDATE_H
+#define _LINUX_LIVEUPDATE_H
+
+#include <linux/bug.h>
+#include <linux/list.h>
+#include <linux/types.h>
+
+#ifdef CONFIG_LIVEUPDATE
+
+/* Return true if live update orchestrator is enabled */
+bool liveupdate_enabled(void);
+
+/* Called during kexec to tell LUO that entered into reboot */
+int liveupdate_reboot(void);
+
+#else /* CONFIG_LIVEUPDATE */
+
+static inline bool liveupdate_enabled(void)
+{
+	return false;
+}
+
+static inline int liveupdate_reboot(void)
+{
+	return 0;
+}
+
+#endif /* CONFIG_LIVEUPDATE */
+#endif /* _LINUX_LIVEUPDATE_H */

include/uapi/linux/liveupdate.h

···
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+
+/*
+ * Userspace interface for /dev/liveupdate
+ * Live Update Orchestrator
+ *
+ * Copyright (c) 2025, Google LLC.
+ * Pasha Tatashin <[e-mail]>
+ */
+
+#ifndef _UAPI_LIVEUPDATE_H
+#define _UAPI_LIVEUPDATE_H
+
+#include <linux/ioctl.h>
+#include <linux/types.h>
+
+/**
+ * DOC: General ioctl format
+ *
+ * The ioctl interface follows a general format to allow for extensibility. Each
+ * ioctl is passed in a structure pointer as the argument providing the size of
+ * the structure in the first u32. The kernel checks that any structure space
+ * beyond what it understands is 0. This allows userspace to use the backward
+ * compatible portion while consistently using the newer, larger, structures.
+ *
+ * ioctls use a standard meaning for common errnos:
+ *
+ *  - ENOTTY: The IOCTL number itself is not supported at all
+ *  - E2BIG: The IOCTL number is supported, but the provided structure has
+ *    non-zero in a part the kernel does not understand.
+ *  - EOPNOTSUPP: The IOCTL number is supported, and the structure is
+ *    understood, however a known field has a value the kernel does not
+ *    understand or support.
+ *  - EINVAL: Everything about the IOCTL was understood, but a field is not
+ *    correct.
+ *  - ENOENT: A provided token does not exist.
+ *  - ENOMEM: Out of memory.
+ *  - EOVERFLOW: Mathematics overflowed.
+ *
+ * As well as additional errnos, within specific ioctls.
+ */
+
+/* The ioctl type, documented in ioctl-number.rst */
+#define LIVEUPDATE_IOCTL_TYPE		0xBA
+
+#endif /* _UAPI_LIVEUPDATE_H */

kernel/liveupdate/Kconfig

···
 	  The default behavior can still be overridden at boot time by
 	  passing 'kho=off'.
 
+config LIVEUPDATE
+	bool "Live Update Orchestrator"
+	depends on KEXEC_HANDOVER
+	help
+	  Enable the Live Update Orchestrator. Live Update is a mechanism,
+	  typically based on kexec, that allows the kernel to be updated
+	  while keeping selected devices operational across the transition.
+	  These devices are intended to be reclaimed by the new kernel and
+	  re-attached to their original workload without requiring a device
+	  reset.
+
+	  Ability to handover a device from current to the next kernel depends
+	  on specific support within device drivers and related kernel
+	  subsystems.
+
+	  This feature primarily targets virtual machine hosts to quickly update
+	  the kernel hypervisor with minimal disruption to the running virtual
+	  machines.
+
+	  If unsure, say N.
+
 endmenu

kernel/liveupdate/Makefile

···
 # SPDX-License-Identifier: GPL-2.0
 
+luo-y :=								\
+		luo_core.o
+
 obj-$(CONFIG_KEXEC_HANDOVER)		+= kexec_handover.o
 obj-$(CONFIG_KEXEC_HANDOVER_DEBUG)	+= kexec_handover_debug.o
 obj-$(CONFIG_KEXEC_HANDOVER_DEBUGFS)	+= kexec_handover_debugfs.o
+
+obj-$(CONFIG_LIVEUPDATE)		+= luo.o

kernel/liveupdate/luo_core.c

···
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Copyright (c) 2025, Google LLC.
+ * Pasha Tatashin <[e-mail]>
+ */
+
+/**
+ * DOC: Live Update Orchestrator (LUO)
+ *
+ * Live Update is a specialized, kexec-based reboot process that allows a
+ * running kernel to be updated from one version to another while preserving
+ * the state of selected resources and keeping designated hardware devices
+ * operational. For these devices, DMA activity may continue throughout the
+ * kernel transition.
+ *
+ * While the primary use case driving this work is supporting live updates of
+ * the Linux kernel when it is used as a hypervisor in cloud environments, the
+ * LUO framework itself is designed to be workload-agnostic. Live Update
+ * facilitates a full kernel version upgrade for any type of system.
+ *
+ * For example, a non-hypervisor system running an in-memory cache like
+ * memcached with many gigabytes of data can use LUO. The userspace service
+ * can place its cache into a memfd, have its state preserved by LUO, and
+ * restore it immediately after the kernel kexec.
+ *
+ * Whether the system is running virtual machines, containers, a
+ * high-performance database, or networking services, LUO's primary goal is to
+ * enable a full kernel update by preserving critical userspace state and
+ * keeping essential devices operational.
+ *
+ * The core of LUO is a mechanism that tracks the progress of a live update,
+ * along with a callback API that allows other kernel subsystems to participate
+ * in the process. Example subsystems that can hook into LUO include: kvm,
+ * iommu, interrupts, vfio, participating filesystems, and memory management.
+ *
+ * LUO uses Kexec Handover to transfer memory state from the current kernel to
+ * the next kernel. For more details see
+ * Documentation/core-api/kho/concepts.rst.
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/kobject.h>
+#include <linux/liveupdate.h>
+#include <linux/miscdevice.h>
+
+static struct {
+	bool enabled;
+} luo_global;
+
+static int __init early_liveupdate_param(char *buf)
+{
+	return kstrtobool(buf, &luo_global.enabled);
+}
+early_param("liveupdate", early_liveupdate_param);
+
+/* Public Functions */
+
+/**
+ * liveupdate_reboot() - Kernel reboot notifier for live update final
+ * serialization.
+ *
+ * This function is invoked directly from the reboot() syscall pathway
+ * if kexec is in progress.
+ *
+ * If any callback fails, this function aborts KHO, undoes the freeze()
+ * callbacks, and returns an error.
+ */
+int liveupdate_reboot(void)
+{
+	return 0;
+}
+
+/**
+ * liveupdate_enabled - Check if the live update feature is enabled.
+ *
+ * This function returns the state of the live update feature flag, which
+ * can be controlled via the ``liveupdate`` kernel command-line parameter.
+ *
+ * @return true if live update is enabled, false otherwise.
+ */
+bool liveupdate_enabled(void)
+{
+	return luo_global.enabled;
+}
+
+struct luo_device_state {
+	struct miscdevice miscdev;
+};
+
+static const struct file_operations luo_fops = {
+	.owner		= THIS_MODULE,
+};
+
+static struct luo_device_state luo_dev = {
+	.miscdev = {
+		.minor = MISC_DYNAMIC_MINOR,
+		.name  = "liveupdate",
+		.fops  = &luo_fops,
+	},
+};
+
+static int __init liveupdate_ioctl_init(void)
+{
+	if (!liveupdate_enabled())
+		return 0;
+
+	return misc_register(&luo_dev.miscdev);
+}
+late_initcall(liveupdate_ioctl_init);