netdev - [PATCH v10 3/3] vhost: Add new UAPI to select kthread mode and KConfig to enable this IOCTL

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Message-ID: <20250531095135.159582-4-lulu@redhat.com>
Date: Sat, 31 May 2025 17:47:05 +0800
From: Cindy Lu <lulu@...hat.com>
To: lulu@...hat.com,
	jasowang@...hat.com,
	mst@...hat.com,
	michael.christie@...cle.com,
	sgarzare@...hat.com,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	netdev@...r.kernel.org
Subject: [PATCH v10 3/3] vhost: Add new UAPI to select kthread mode and KConfig to enable this IOCTL

This patch introduces a new UAPI that allows the vhost device to select
in kthread mode. Userspace applications can utilize IOCTL
VHOST_FORK_FROM_OWNER to select between task and kthread modes, which
must be invoked before IOCTL VHOST_SET_OWNER, as the worker will be
created during that call.

The VHOST_NEW_WORKER requires the inherit_owner setting to be true, and
a check has been added to ensure proper configuration.

Additionally, a new KConfig option, CONFIG_VHOST_ENABLE_FORK_OWNER_IOCTL,
is introduced to control the availability of the IOCTL
VHOST_FORK_FROM_OWNER. When CONFIG_VHOST_ENABLE_FORK_OWNER_IOCTL is set
to n, the IOCTL is disabled, and any attempt to use it will result in a
failure.

Signed-off-by: Cindy Lu <lulu@...hat.com>
---
 drivers/vhost/Kconfig      | 13 +++++++++++++
 drivers/vhost/vhost.c      | 30 +++++++++++++++++++++++++++++-
 include/uapi/linux/vhost.h | 16 ++++++++++++++++
 3 files changed, 58 insertions(+), 1 deletion(-)

diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
index 020d4fbb947c..300e474b60fd 100644
--- a/drivers/vhost/Kconfig
+++ b/drivers/vhost/Kconfig
@@ -96,3 +96,16 @@ config VHOST_CROSS_ENDIAN_LEGACY
 	  If unsure, say "N".
 
 endif
+
+config VHOST_ENABLE_FORK_OWNER_IOCTL
+	bool "Enable IOCTL VHOST_FORK_FROM_OWNER"
+	default n
+	help
+	  This option enables the IOCTL VHOST_FORK_FROM_OWNER, allowing
+	  userspace applications to modify the thread mode for vhost devices.
+
+	  By default, `CONFIG_VHOST_ENABLE_FORK_OWNER_IOCTL` is set to `n`,
+	  which disables the IOCTL. When enabled (y), the IOCTL allows
+	  users to set the mode as needed.
+
+	  If unsure, say "N".
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 2d2909be1bb2..673c2dc8607b 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -1022,6 +1022,13 @@ 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 inherit_owner must be true.
+		 */
+		if (!dev->inherit_owner)
+			return -EFAULT;
 		ret = vhost_new_worker(dev, &state);
 		if (!ret && copy_to_user(argp, &state, sizeof(state)))
 			ret = -EFAULT;
@@ -1138,7 +1145,7 @@ void vhost_dev_reset_owner(struct vhost_dev *dev, struct vhost_iotlb *umem)
 	int i;
 
 	vhost_dev_cleanup(dev);
-
+	dev->inherit_owner = true;
 	dev->umem = umem;
 	/* We don't need VQ locks below since vhost_dev_cleanup makes sure
 	 * VQs aren't running.
@@ -2292,6 +2299,27 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
 		goto done;
 	}
 
+#ifdef CONFIG_VHOST_ENABLE_FORK_OWNER_IOCTL
+	if (ioctl == VHOST_FORK_FROM_OWNER) {
+		u8 inherit_owner;
+		/*inherit_owner can only be modified before owner is set*/
+		if (vhost_dev_has_owner(d)) {
+			r = -EBUSY;
+			goto done;
+		}
+		if (copy_from_user(&inherit_owner, argp, sizeof(u8))) {
+			r = -EFAULT;
+			goto done;
+		}
+		if (inherit_owner > 1) {
+			r = -EINVAL;
+			goto done;
+		}
+		d->inherit_owner = (bool)inherit_owner;
+		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..d2692c7ef450 100644
--- a/include/uapi/linux/vhost.h
+++ b/include/uapi/linux/vhost.h
@@ -235,4 +235,20 @@
  */
 #define VHOST_VDPA_GET_VRING_SIZE	_IOWR(VHOST_VIRTIO, 0x82,	\
 					      struct vhost_vring_state)
+
+/**
+ * VHOST_FORK_FROM_OWNER - Set the inherit_owner flag for the vhost device,
+ * This ioctl must called before VHOST_SET_OWNER.
+ *
+ * @param inherit_owner: An 8-bit value that determines the vhost thread mode
+ *
+ * When inherit_owner is set to 1(default value):
+ *   - Vhost will create tasks similar to processes forked from the owner,
+ *     inheriting all of the owner's attributes.
+ *
+ * When inherit_owner is set to 0:
+ *   - Vhost will create tasks as kernel thread.
+ */
+#define VHOST_FORK_FROM_OWNER _IOW(VHOST_VIRTIO, 0x83, __u8)
+
 #endif
-- 
2.45.0