[PATCH v11 3/3] vhost: Add configuration controls for vhost worker's mode

[Cindy Lu <lulu@redhat.com>]

This patch introduces functionality to control the vhost worker mode:

- Add two new IOCTLs:
  * VHOST_SET_FORK_FROM_OWNER: Allows userspace to select between
    task mode (fork_owner=1) and kthread mode (fork_owner=0)
  * VHOST_GET_FORK_FROM_OWNER: Retrieves the current thread mode
    setting

- Expose module parameter 'fork_from_owner_default' to allow system
  administrators to configure the default mode for vhost workers

- Add KConfig option CONFIG_VHOST_ENABLE_FORK_OWNER_CONTROL to
  control the availability of these IOCTLs and parameter, allowing
  distributions to disable them if not needed

- The VHOST_NEW_WORKER functionality requires fork_owner to be set
  to true, with validation added to ensure proper configuration

Signed-off-by: Cindy Lu <lulu@redhat.com>
---
 drivers/vhost/Kconfig      | 17 +++++++++++++++
 drivers/vhost/vhost.c      | 44 ++++++++++++++++++++++++++++++++++++++
 include/uapi/linux/vhost.h | 25 ++++++++++++++++++++++
 3 files changed, 86 insertions(+)

diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
index 020d4fbb947c..49e1d9dc92b7 100644
--- a/drivers/vhost/Kconfig
+++ b/drivers/vhost/Kconfig
@@ -96,3 +96,20 @@ config VHOST_CROSS_ENDIAN_LEGACY
 	  If unsure, say "N".
 
 endif
+
+config CONFIG_VHOST_ENABLE_FORK_OWNER_CONTROL
+	bool "Enable CONFIG_VHOST_ENABLE_FORK_OWNER_CONTROL"
+	default n
+	help
+	  This option enables two IOCTLs: VHOST_SET_FORK_FROM_OWNER and
+	  VHOST_GET_FORK_FROM_OWNER. These allow userspace applications
+	  to modify the vhost worker mode for vhost devices.
+
+	  Also expose module parameter 'fork_from_owner_default' to allow users
+	  to configure the default mode for vhost workers.
+
+	  By default, `CONFIG_VHOST_ENABLE_FORK_OWNER_CONTROL` is set to `n`,
+	  which disables the IOCTLs and parameter.
+	  When enabled (y), users can change the worker thread mode as needed.
+
+	  If unsure, say "N".
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 37d3ed8be822..903d9c3f6784 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -43,6 +43,11 @@ module_param(max_iotlb_entries, int, 0444);
 MODULE_PARM_DESC(max_iotlb_entries,
 	"Maximum number of iotlb entries. (default: 2048)");
 static bool fork_from_owner_default = true;
+#ifdef CONFIG_VHOST_ENABLE_FORK_OWNER_CONTROL
+module_param(fork_from_owner_default, bool, 0444);
+MODULE_PARM_DESC(fork_from_owner_default,
+		 "Set task mode as the default(default: Y)");
+#endif
 
 enum {
 	VHOST_MEMORY_F_LOG = 0x1,
@@ -1019,6 +1024,13 @@ long vhost_worker_ioctl(struct vhost_dev *dev, unsigned int ioctl,
 	switch (ioctl) {
 	/* dev worker ioctls */
 	case VHOST_NEW_WORKER:
+		/*
+		 * vhost_tasks will account for worker threads under the parent's
+		 * NPROC value but kthreads do not. To avoid userspace overflowing
+		 * the system with worker threads fork_owner must be true.
+		 */
+		if (!dev->fork_owner)
+			return -EFAULT;
 		ret = vhost_new_worker(dev, &state);
 		if (!ret && copy_to_user(argp, &state, sizeof(state)))
 			ret = -EFAULT;
@@ -1136,6 +1148,7 @@ void vhost_dev_reset_owner(struct vhost_dev *dev, struct vhost_iotlb *umem)
 
 	vhost_dev_cleanup(dev);
 
+	dev->fork_owner = fork_from_owner_default;
 	dev->umem = umem;
 	/* We don't need VQ locks below since vhost_dev_cleanup makes sure
 	 * VQs aren't running.
@@ -2289,6 +2302,37 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
 		goto done;
 	}
 
+#ifdef CONFIG_VHOST_ENABLE_FORK_OWNER_CONTROL
+	u8 fork_owner;
+
+	if (ioctl == VHOST_SET_FORK_FROM_OWNER) {
+		/*fork_owner can only be modified before owner is set*/
+		if (vhost_dev_has_owner(d)) {
+			r = -EBUSY;
+			goto done;
+		}
+		if (copy_from_user(&fork_owner, argp, sizeof(u8))) {
+			r = -EFAULT;
+			goto done;
+		}
+		if (fork_owner > 1) {
+			r = -EINVAL;
+			goto done;
+		}
+		d->fork_owner = (bool)fork_owner;
+		r = 0;
+		goto done;
+	}
+	if (ioctl == VHOST_GET_FORK_FROM_OWNER) {
+		fork_owner = d->fork_owner;
+		if (copy_to_user(argp, &fork_owner, sizeof(u8))) {
+			r = -EFAULT;
+			goto done;
+		}
+		r = 0;
+		goto done;
+	}
+#endif
 	/* You must be the owner to do anything else */
 	r = vhost_dev_check_owner(d);
 	if (r)
diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
index d4b3e2ae1314..e51d6a347607 100644
--- a/include/uapi/linux/vhost.h
+++ b/include/uapi/linux/vhost.h
@@ -235,4 +235,29 @@
  */
 #define VHOST_VDPA_GET_VRING_SIZE	_IOWR(VHOST_VIRTIO, 0x82,	\
 					      struct vhost_vring_state)
+
+/**
+ * VHOST_SET_FORK_FROM_OWNER - Set the fork_owner flag for the vhost device,
+ * This ioctl must called before VHOST_SET_OWNER.
+ * Only available when CONFIG_VHOST_ENABLE_FORK_OWNER_CONTROL=y
+ *
+ * @param fork_owner: An 8-bit value that determines the vhost thread mode
+ *
+ * When fork_owner is set to 1(default value):
+ *   - Vhost will create vhost worker as tasks forked from the owner,
+ *     inheriting all of the owner's attributes.
+ *
+ * When fork_owner is set to 0:
+ *   - Vhost will create vhost workers as kernel threads.
+ */
+#define VHOST_SET_FORK_FROM_OWNER _IOW(VHOST_VIRTIO, 0x83, __u8)
+
+/**
+ * VHOST_GET_FORK_OWNER - Get the current fork_owner flag for the vhost device.
+ * Only available when CONFIG_VHOST_ENABLE_FORK_OWNER_CONTROL=y
+ *
+ * @return: An 8-bit value indicating the current thread mode.
+ */
+#define VHOST_GET_FORK_FROM_OWNER _IOR(VHOST_VIRTIO, 0x84, __u8)
+
 #endif
-- 
2.45.0