[PATCH v3 19/30] liveupdate: luo_sysfs: add sysfs state monitoring

[Pasha Tatashin <pasha.tatashin@soleen.com>]

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 <pasha.tatashin@soleen.com>
---
 .../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 <pasha.tatashin@soleen.com>
+ */
+
+/**
+ * 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 <linux/kobject.h>
+#include <linux/liveupdate.h>
+#include <linux/sysfs.h>
+#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