VDUSE (vDPA Device in Userspace) is a framework to support
implementing software-emulated vDPA devices in userspace. This
document is intended to clarify the VDUSE design and usage.

Signed-off-by: Xie Yongji <redacted>
---
  Documentation/userspace-api/index.rst |   1 +
  Documentation/userspace-api/vduse.rst | 232 ++++++++++++++++++++++++++++++++++
  2 files changed, 233 insertions(+)
  create mode 100644 Documentation/userspace-api/vduse.rst

diff --git a/Documentation/userspace-api/index.rst b/Documentation/userspace-api/index.rst
index 0b5eefed027e..c432be070f67 100644
--- a/Documentation/userspace-api/index.rst
+++ b/Documentation/userspace-api/index.rst

@@ -27,6 +27,7 @@ place where this information is gathered.
     iommu
     media/index
     sysfs-platform_profile
+   vduse
  
  .. only::  subproject and html
  

diff --git a/Documentation/userspace-api/vduse.rst b/Documentation/userspace-api/vduse.rst
new file mode 100644
index 000000000000..30c9d1482126
--- /dev/null
+++ b/Documentation/userspace-api/vduse.rst

@@ -0,0 +1,232 @@
+==================================
+VDUSE - "vDPA Device in Userspace"
+==================================
+
+vDPA (virtio data path acceleration) device is a device that uses a
+datapath which complies with the virtio specifications with vendor
+specific control path. vDPA devices can be both physically located on
+the hardware or emulated by software. VDUSE is a framework that makes it
+possible to implement software-emulated vDPA devices in userspace. And
+to make the device emulation more secure, the emulated vDPA device's
+control path is handled in the kernel and only the data path is
+implemented in the userspace.
+
+Note that only virtio block device is supported by VDUSE framework now,
+which can reduce security risks when the userspace process that implements
+the data path is run by an unprivileged user. The support for other device
+types can be added after the security issue of corresponding device driver
+is clarified or fixed in the future.
+
+Create/Destroy VDUSE devices
+------------------------
+
+VDUSE devices are created as follows:
+
+1. Create a new VDUSE instance with ioctl(VDUSE_CREATE_DEV) on
+   /dev/vduse/control.
+
+2. Setup each virtqueue with ioctl(VDUSE_VQ_SETUP) on /dev/vduse/$NAME.
+
+3. Begin processing VDUSE messages from /dev/vduse/$NAME. The first
+   messages will arrive while attaching the VDUSE instance to vDPA bus.
+
+4. Send the VDPA_CMD_DEV_NEW netlink message to attach the VDUSE
+   instance to vDPA bus.
+
+VDUSE devices are destroyed as follows:
+
+1. Send the VDPA_CMD_DEV_DEL netlink message to detach the VDUSE
+   instance from vDPA bus.
+
+2. Close the file descriptor referring to /dev/vduse/$NAME.
+
+3. Destroy the VDUSE instance with ioctl(VDUSE_DESTROY_DEV) on
+   /dev/vduse/control.
+
+The netlink messages can be sent via vdpa tool in iproute2 or use the
+below sample codes:
+
+.. code-block:: c
+
+	static int netlink_add_vduse(const char *name, enum vdpa_command cmd)
+	{
+		struct nl_sock *nlsock;
+		struct nl_msg *msg;
+		int famid;
+
+		nlsock = nl_socket_alloc();
+		if (!nlsock)
+			return -ENOMEM;
+
+		if (genl_connect(nlsock))
+			goto free_sock;
+
+		famid = genl_ctrl_resolve(nlsock, VDPA_GENL_NAME);
+		if (famid < 0)
+			goto close_sock;
+
+		msg = nlmsg_alloc();
+		if (!msg)
+			goto close_sock;
+
+		if (!genlmsg_put(msg, NL_AUTO_PORT, NL_AUTO_SEQ, famid, 0, 0, cmd, 0))
+			goto nla_put_failure;
+
+		NLA_PUT_STRING(msg, VDPA_ATTR_DEV_NAME, name);
+		if (cmd == VDPA_CMD_DEV_NEW)
+			NLA_PUT_STRING(msg, VDPA_ATTR_MGMTDEV_DEV_NAME, "vduse");
+
+		if (nl_send_sync(nlsock, msg))
+			goto close_sock;
+
+		nl_close(nlsock);
+		nl_socket_free(nlsock);
+
+		return 0;
+	nla_put_failure:
+		nlmsg_free(msg);
+	close_sock:
+		nl_close(nlsock);
+	free_sock:
+		nl_socket_free(nlsock);
+		return -1;
+	}
+
+How VDUSE works
+---------------
+
+As mentioned above, a VDUSE device is created by ioctl(VDUSE_CREATE_DEV) on
+/dev/vduse/control. With this ioctl, userspace can specify some basic configuration
+such as device name (uniquely identify a VDUSE device), virtio features, virtio
+configuration space, the number of virtqueues and so on for this emulated device.
+Then a char device interface (/dev/vduse/$NAME) is exported to userspace for device
+emulation. Userspace can use the VDUSE_VQ_SETUP ioctl on /dev/vduse/$NAME to
+add per-virtqueue configuration such as the max size of virtqueue to the device.
+
+After the initialization, the VDUSE device can be attached to vDPA bus via
+the VDPA_CMD_DEV_NEW netlink message. Userspace needs to read()/write() on
+/dev/vduse/$NAME to receive/reply some control messages from/to VDUSE kernel
+module as follows:
+
+.. code-block:: c
+
+	static int vduse_message_handler(int dev_fd)
+	{
+		int len;
+		struct vduse_dev_request req;
+		struct vduse_dev_response resp;
+
+		len = read(dev_fd, &req, sizeof(req));
+		if (len != sizeof(req))
+			return -1;
+
+		resp.request_id = req.request_id;
+
+		switch (req.type) {
+
+		/* handle different types of messages */
+
+		}
+
+		len = write(dev_fd, &resp, sizeof(resp));
+		if (len != sizeof(resp))
+			return -1;
+
+		return 0;
+	}
+
+There are now three types of messages introduced by VDUSE framework:
+
+- VDUSE_GET_VQ_STATE: Get the state for virtqueue, userspace should return
+  avail index for split virtqueue or the device/driver ring wrap counters and
+  the avail and used index for packed virtqueue.
+
+- VDUSE_SET_STATUS: Set the device status, userspace should follow
+  the virtio spec: https://docs.oasis-open.org/virtio/virtio/v1.1/virtio-v1.1.html
+  to process this message. For example, fail to set the FEATURES_OK device
+  status bit if the device can not accept the negotiated virtio features
+  get from the VDUSE_DEV_GET_FEATURES ioctl.