Introduce a sysfs interface for the Live Update Orchestrator under /sys/kernel/liveupdate/. This interface provides a way for userspace tools and scripts to monitor the current state of the LUO state machine. The main feature is a read-only file, state, which displays the current LUO state as a string ("normal", "prepared", "frozen", "updated"). The interface uses sysfs_notify to allow userspace listeners (e.g., via poll) to be efficiently notified of state changes. ABI documentation for this new sysfs interface is added in Documentation/ABI/testing/sysfs-kernel-liveupdate. This read-only sysfs interface complements the main ioctl interface provided by /dev/liveupdate, which handles LUO control operations and resource management. Signed-off-by: Pasha Tatashin --- .../ABI/testing/sysfs-kernel-liveupdate | 51 ++++++++++ kernel/liveupdate/Kconfig | 18 ++++ kernel/liveupdate/Makefile | 1 + kernel/liveupdate/luo_core.c | 1 + kernel/liveupdate/luo_internal.h | 6 ++ kernel/liveupdate/luo_sysfs.c | 92 +++++++++++++++++++ 6 files changed, 169 insertions(+) create mode 100644 Documentation/ABI/testing/sysfs-kernel-liveupdate create mode 100644 kernel/liveupdate/luo_sysfs.c diff --git a/Documentation/ABI/testing/sysfs-kernel-liveupdate b/Documentation/ABI/testing/sysfs-kernel-liveupdate new file mode 100644 index 000000000000..bb85cbae4943 --- /dev/null +++ b/Documentation/ABI/testing/sysfs-kernel-liveupdate @@ -0,0 +1,51 @@ +What: /sys/kernel/liveupdate/ +Date: May 2025 +KernelVersion: 6.16.0 +Contact: pasha.tatashin@soleen.com +Description: Directory containing interfaces to query the live + update orchestrator. Live update is the ability to reboot the + host kernel (e.g., via kexec, without a full power cycle) while + keeping specifically designated devices operational ("alive") + across the transition. After the new kernel boots, these devices + can be re-attached to their original workloads (e.g., virtual + machines) with their state preserved. This is particularly + useful, for example, for quick hypervisor updates without + terminating running virtual machines. + + +What: /sys/kernel/liveupdate/state +Date: May 2025 +KernelVersion: 6.16.0 +Contact: pasha.tatashin@soleen.com +Description: Read-only file that displays the current state of the live + update orchestrator as a string. Possible values are: + + "normal" No live update operation is in progress. This is + the default operational state. + + "prepared" The live update preparation phase has completed + successfully (e.g., triggered via the + /dev/liveupdate event). Kernel subsystems have + been notified via the %LIVEUPDATE_PREPARE + event/callback and should have initiated state + saving. User workloads (e.g., VMs) are generally + still running, but some operations (like device + unbinding or new DMA mappings) might be + restricted. The system is ready for the reboot + trigger. + + "frozen" The final reboot notification has been sent + (e.g., triggered via the 'reboot()' syscall), + corresponding to the %LIVEUPDATE_REBOOT kernel + event. Subsystems have had their final chance to + save state. User workloads must be suspended. + The system is about to execute the reboot into + the new kernel (imminent kexec). This state + corresponds to the "blackout window". + + "updated" The system has successfully rebooted into the + new kernel via live update. Restoration of + preserved resources can now occur (typically via + ioctl commands). The system is awaiting the + final 'finish' signal after user space completes + restoration tasks. diff --git a/kernel/liveupdate/Kconfig b/kernel/liveupdate/Kconfig index f6b0bde188d9..75a17ca8a592 100644 --- a/kernel/liveupdate/Kconfig +++ b/kernel/liveupdate/Kconfig @@ -29,6 +29,24 @@ config LIVEUPDATE If unsure, say N. +config LIVEUPDATE_SYSFS_API + bool "Live Update sysfs monitoring interface" + depends on SYSFS + depends on LIVEUPDATE + help + Enable a sysfs interface for the Live Update Orchestrator + at /sys/kernel/liveupdate/. + + This allows monitoring the LUO state ('normal', 'prepared', + 'frozen', 'updated') via the read-only 'state' file. + + This interface complements the primary /dev/liveupdate ioctl + interface, which handles the full update process. + This sysfs API may be useful for scripting, or userspace monitoring + needed to coordinate application restarts and minimize downtime. + + If unsure, say N. + config KEXEC_HANDOVER bool "kexec handover" depends on ARCH_SUPPORTS_KEXEC_HANDOVER && ARCH_SUPPORTS_KEXEC_FILE diff --git a/kernel/liveupdate/Makefile b/kernel/liveupdate/Makefile index c67fa2797796..47f5d0378a75 100644 --- a/kernel/liveupdate/Makefile +++ b/kernel/liveupdate/Makefile @@ -13,3 +13,4 @@ obj-$(CONFIG_KEXEC_HANDOVER) += kexec_handover.o obj-$(CONFIG_KEXEC_HANDOVER_DEBUG) += kexec_handover_debug.o obj-$(CONFIG_LIVEUPDATE) += luo.o +obj-$(CONFIG_LIVEUPDATE_SYSFS_API) += luo_sysfs.o diff --git a/kernel/liveupdate/luo_core.c b/kernel/liveupdate/luo_core.c index 64d53b31d6d8..bd07ee859112 100644 --- a/kernel/liveupdate/luo_core.c +++ b/kernel/liveupdate/luo_core.c @@ -100,6 +100,7 @@ static inline bool is_current_luo_state(enum liveupdate_state expected_state) static void __luo_set_state(enum liveupdate_state state) { WRITE_ONCE(luo_state, state); + luo_sysfs_notify(); } static inline void luo_set_state(enum liveupdate_state state) diff --git a/kernel/liveupdate/luo_internal.h b/kernel/liveupdate/luo_internal.h index 01bd0d3b023b..9091ed04c606 100644 --- a/kernel/liveupdate/luo_internal.h +++ b/kernel/liveupdate/luo_internal.h @@ -47,4 +47,10 @@ int luo_file_freeze(u64 token); int luo_file_cancel(u64 token); int luo_file_finish(u64 token); +#ifdef CONFIG_LIVEUPDATE_SYSFS_API +void luo_sysfs_notify(void); +#else +static inline void luo_sysfs_notify(void) {} +#endif + #endif /* _LINUX_LUO_INTERNAL_H */ diff --git a/kernel/liveupdate/luo_sysfs.c b/kernel/liveupdate/luo_sysfs.c new file mode 100644 index 000000000000..935946bb741b --- /dev/null +++ b/kernel/liveupdate/luo_sysfs.c @@ -0,0 +1,92 @@ +// SPDX-License-Identifier: GPL-2.0 + +/* + * Copyright (c) 2025, Google LLC. + * Pasha Tatashin + */ + +/** + * DOC: LUO sysfs interface + * + * Provides a sysfs interface at ``/sys/kernel/liveupdate/`` for monitoring LUO + * state. Live update allows rebooting the kernel (via kexec) while preserving + * designated device state for attached workloads (e.g., VMs), useful for + * minimizing downtime during hypervisor updates. + * + * /sys/kernel/liveupdate/state + * ---------------------------- + * - Permissions: Read-only + * - Description: Displays the current LUO state string. + * - Valid States: + * @normal + * Idle state. + * @prepared + * Preparation phase complete (triggered via '/dev/liveupdate'). Resources + * checked, state saving initiated via %LIVEUPDATE_PREPARE event. + * Workloads mostly running but may be restricted. Ready forreboot + * trigger. + * @frozen + * Final reboot notification sent (triggered via 'reboot'). Corresponds to + * %LIVEUPDATE_REBOOT event. Final state saving. Workloads must be + * suspended. System about to kexec ("blackout window"). + * @updated + * New kernel booted via live update. Awaiting 'finish' signal. + * + * Userspace Interaction & Blackout Window Reduction + * ------------------------------------------------- + * Userspace monitors the ``state`` file to coordinate actions: + * - Suspend workloads before @frozen state is entered. + * - Initiate resource restoration upon entering @updated state. + * - Resume workloads after restoration, minimizing downtime. + */ + +#include +#include +#include +#include "luo_internal.h" + +static bool luo_sysfs_initialized; + +#define LUO_DIR_NAME "liveupdate" + +void luo_sysfs_notify(void) +{ + if (luo_sysfs_initialized) + sysfs_notify(kernel_kobj, LUO_DIR_NAME, "state"); +} + +/* Show the current live update state */ +static ssize_t state_show(struct kobject *kobj, struct kobj_attribute *attr, + char *buf) +{ + return sysfs_emit(buf, "%s\n", luo_current_state_str()); +} + +static struct kobj_attribute state_attribute = __ATTR_RO(state); + +static struct attribute *luo_attrs[] = { + &state_attribute.attr, + NULL +}; + +static struct attribute_group luo_attr_group = { + .attrs = luo_attrs, + .name = LUO_DIR_NAME, +}; + +static int __init luo_init(void) +{ + int ret; + + ret = sysfs_create_group(kernel_kobj, &luo_attr_group); + if (ret) { + pr_err("Failed to create group\n"); + return ret; + } + + luo_sysfs_initialized = true; + pr_info("Initialized\n"); + + return 0; +} +subsys_initcall(luo_init); -- 2.50.1.565.gc32cd1483b-goog