[PATCH v6 12/23] virt: Introduce steal monitor driver

[Shrikanth Hegde <sshegde@linux.ibm.com>]

Introduce a new driver in virt named steal_monitor. This driver
will compute the steal time and drive the policy decisions of preferred
CPU state.

More on it can be found in the Documentation/driver-api/steal-monitor.rst
Introduce the skeleton code first.

There is no new kconfig. It depends on CONFIG_PREFERRED_CPU.
- If CONFIG_PREFERRED_CPU=y, it gets compiled as a module. It is not
  loaded by default.
- If CONFIG_PREFERRED_CPU=n, module isn't compiled.

File layout of the driver is designed with having arch specific
files in the future.

- sm_core.c - contains main driver code. This includes the periodic
  work function and take action on steal time.
- defaults.c - contains the default implementation defined with __weak
  symbols.
- sm_core.h - header file which includes data structure.

Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
---
 drivers/virt/Makefile                |  1 +
 drivers/virt/steal_monitor/Makefile  | 14 ++++++++++++
 drivers/virt/steal_monitor/sm_core.c | 33 ++++++++++++++++++++++++++++
 drivers/virt/steal_monitor/sm_core.h | 11 ++++++++++
 4 files changed, 59 insertions(+)
 create mode 100644 drivers/virt/steal_monitor/Makefile
 create mode 100644 drivers/virt/steal_monitor/sm_core.c
 create mode 100644 drivers/virt/steal_monitor/sm_core.h

diff --git a/drivers/virt/Makefile b/drivers/virt/Makefile
index f29901bd7820..aff715cea42d 100644
--- a/drivers/virt/Makefile
+++ b/drivers/virt/Makefile
@@ -9,4 +9,5 @@ obj-y				+= vboxguest/
 
 obj-$(CONFIG_NITRO_ENCLAVES)	+= nitro_enclaves/
 obj-$(CONFIG_ACRN_HSM)		+= acrn/
+obj-$(CONFIG_PREFERRED_CPU)	+= steal_monitor/
 obj-y				+= coco/
diff --git a/drivers/virt/steal_monitor/Makefile b/drivers/virt/steal_monitor/Makefile
new file mode 100644
index 000000000000..24cee55342ce
--- /dev/null
+++ b/drivers/virt/steal_monitor/Makefile
@@ -0,0 +1,14 @@
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# Steal time monitor to alter preferred CPU state.
+#
+# Arch can implement strong function definitions and override the
+# default by adding them in arch specific file. It must ensure
+# that preferred is always subset of active.
+#
+# It is always compiled as module if CONFIG_PREFERRED_CPU=y
+# One has to enable the module.
+#
+obj-$(subst y,m,$(CONFIG_PREFERRED_CPU)) += steal_monitor.o
+
+steal_monitor-y := sm_core.o
diff --git a/drivers/virt/steal_monitor/sm_core.c b/drivers/virt/steal_monitor/sm_core.c
new file mode 100644
index 000000000000..e320559c6576
--- /dev/null
+++ b/drivers/virt/steal_monitor/sm_core.c
@@ -0,0 +1,33 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Steal time Monitor.
+ *
+ * Periodically compute steal time. Based on the thresholds either
+ * reduce/increase the preferred CPUs which can be made use
+ * by the workload to avoid vCPU preemption to an extent possible.
+ *
+ * Available as module with CONFIG_PREFERRED_CPU=y
+ *
+ * Copyright (C) 2026 IBM
+ * Author: Shrikanth Hegde <sshegde@linux.ibm.com>
+ */
+
+#include "sm_core.h"
+
+static int __init steal_monitor_init(void)
+{
+	pr_info("steal_monitor is enabled\n");
+	return 0;
+}
+
+static void __exit steal_monitor_exit(void)
+{
+	pr_info("steal_monitor is disabled\n");
+}
+
+module_init(steal_monitor_init);
+module_exit(steal_monitor_exit);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("IBM Corporation");
+MODULE_DESCRIPTION("Virtualization Steal Time Monitor");
diff --git a/drivers/virt/steal_monitor/sm_core.h b/drivers/virt/steal_monitor/sm_core.h
new file mode 100644
index 000000000000..684a258526e1
--- /dev/null
+++ b/drivers/virt/steal_monitor/sm_core.h
@@ -0,0 +1,11 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef __VIRT_STEAL_CORE_H
+#define __VIRT_STEAL_CORE_H
+
+#include <linux/types.h>
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/init.h>
+
+#endif /* __VIRT_STEAL_CORE_H */
-- 
2.47.3