[RFC PATCH 16/46] liveupdate: cpu_preserve: Add sysfs interface

[Pasha Tatashin]

Expose /sys/devices/system/cpu/preserved (global cpumask of preserved
cores) and /sys/devices/system/cpu/cpuX/preserve (per-CPU preservation
attribute and file descriptor target for LUO sessions), and document
both attributes in Documentation/ABI/testing/sysfs-devices-system-cpu.

Signed-off-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
---
 .../ABI/testing/sysfs-devices-system-cpu      | 20 ++++++++
 kernel/liveupdate/cpu_preserve.c              | 49 +++++++++++++++++++
 2 files changed, 69 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
index 82d10d556cc8..88ab03efd433 100644
--- a/Documentation/ABI/testing/sysfs-devices-system-cpu
+++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
@@ -806,3 +806,23 @@ Date:		Nov 2022
 Contact:	Linux kernel mailing list <linux-kernel@xxxxxxxxxxxxxxx>
 Description:
 		(RO) the list of CPUs that can be brought online.
+
+What:		/sys/devices/system/cpu/preserved
+Date:		March 2026
+Contact:	Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
+Description:
+		(RO) cpumask list of physical CPUs currently isolated and
+		preserved across a host kernel live update. Note the naming
+		distinction from the per-CPU /sys/devices/system/cpu/cpuX/preserve
+		attribute.
+
+What:		/sys/devices/system/cpu/cpuX/preserve
+Date:		March 2026
+Contact:	Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
+Description:
+		(RO) returns 1 if cpuX is currently preserved across a host
+		kernel live update, 0 otherwise. Although read-only, this sysfs
+		node exists to be opened and passed by file descriptor to a Live
+		Update Orchestrator (LUO) session via LIVEUPDATE_SESSION_PRESERVE_FD
+		to request preservation of cpuX across kexec.
+
diff --git a/kernel/liveupdate/cpu_preserve.c b/kernel/liveupdate/cpu_preserve.c
index c5fd3a07ccb5..cc27d1624d29 100644
--- a/kernel/liveupdate/cpu_preserve.c
+++ b/kernel/liveupdate/cpu_preserve.c
@@ -606,3 +606,52 @@ static int __init cpu_preserve_early_init(void)
 	return 0;
 }
 early_initcall(cpu_preserve_early_init);
+static ssize_t preserved_show(struct device *dev,
+			      struct device_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%*pbl\n",
+			  cpumask_pr_args(cpu_get_preserved_mask()));
+}
+static DEVICE_ATTR_RO(preserved);
+
+static ssize_t preserve_show(struct device *dev,
+			     struct device_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%d\n", cpu_is_preserved(dev->id));
+}
+static DEVICE_ATTR_RO(preserve);
+
+static int __init cpu_preserve_sysfs_init(void)
+{
+	struct device *dev_root = bus_get_dev_root(&cpu_subsys);
+	int cpu, ret;
+
+	if (dev_root) {
+		ret = sysfs_create_file(&dev_root->kobj, &dev_attr_preserved.attr);
+		put_device(dev_root);
+		if (ret)
+			pr_warn("Failed to create cpu preserved sysfs attribute: %d\n", ret);
+	}
+
+	for_each_possible_cpu(cpu) {
+		struct device *dev = get_cpu_device(cpu);
+
+		if (!dev && cpu_is_preserved(cpu)) {
+			set_cpu_present(cpu, true);
+			arch_register_cpu(cpu);
+			dev = get_cpu_device(cpu);
+		}
+
+		if (dev) {
+			ret = sysfs_create_file(&dev->kobj, &dev_attr_preserve.attr);
+			if (ret)
+				pr_warn("Failed to create cpu%d preserve sysfs attribute: %d\n",
+					cpu, ret);
+		}
+
+		if (cpu_is_preserved(cpu))
+			set_cpu_present(cpu, false);
+	}
+	return 0;
+}
+late_initcall(cpu_preserve_sysfs_init);
-- 
2.55.0.1082.g2b9226bbc0-goog