[PATCH v7 12/12] Documentation: Add documentation for VDUSE

[Xie Yongji <xieyongji@bytedance.com>]

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 <xieyongji@bytedance.com>
---
 Documentation/userspace-api/index.rst |   1 +
 Documentation/userspace-api/vduse.rst | 243 ++++++++++++++++++++++++++++++++++
 2 files changed, 244 insertions(+)
 create mode 100644 Documentation/userspace-api/vduse.rst

diff --git a/Documentation/userspace-api/index.rst b/Documentation/userspace-api/index.rst
index d29b020e5622..2dc25dd9f2aa 100644
--- a/Documentation/userspace-api/index.rst
+++ b/Documentation/userspace-api/index.rst
@@ -25,6 +25,7 @@ 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..a804be347545
--- /dev/null
+++ b/Documentation/userspace-api/vduse.rst
@@ -0,0 +1,243 @@
+==================================
+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.
+
+In general, the userspace process that emulates the device is able to
+run unprivileged. And to reduce security risks, we only support emulating
+a few vDPA devices by default, including: virtio-net device, virtio-blk
+device, virtio-scsi device and virtio-fs device. Only when a sysadmin trusts
+the userspace process enough, it can relax the limitation with a
+'allow_unsafe_device_emulation' module parameter.
+
+How VDUSE works
+===============
+
+Start/Stop VDUSE devices
+------------------------
+
+VDUSE devices are started as follows:
+
+1. Create a new VDUSE instance with ioctl(VDUSE_CREATE_DEV) on
+   /dev/vduse/control.
+
+2. Begin processing VDUSE messages from /dev/vduse/$NAME. The first
+   messages will arrive while attaching the VDUSE instance to vDPA.
+
+3. Send the VDPA_CMD_DEV_NEW netlink message to attach the VDUSE
+   instance to vDPA.
+
+VDUSE devices are stopped as follows:
+
+1. Send the VDPA_CMD_DEV_DEL netlink message to detach the VDUSE
+   instance to vDPA.
+
+2. Close the file descriptor referring to /dev/vduse/$NAME
+
+3. Destroy the VDUSE instance with ioctl(VDUSE_DESTROY_DEV) on
+   /dev/vduse/control
+
+The netlink messages metioned above 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;
+	}
+
+Emulate VDUSE devices
+---------------------
+
+To emulate a VDUSE device, we always need to implement both control path
+and data path for it.
+
+To implement control path, a message-based communication protocol and some
+types of control messages are introduced in the VDUSE framework:
+
+- VDUSE_SET_VQ_ADDR: Set the vring address of virtqueue.
+
+- VDUSE_SET_VQ_NUM: Set the size of virtqueue
+
+- VDUSE_SET_VQ_READY: Set ready status of virtqueue
+
+- VDUSE_GET_VQ_READY: Get ready status of virtqueue
+
+- VDUSE_SET_VQ_STATE: Set the state for virtqueue
+
+- VDUSE_GET_VQ_STATE: Get the state for virtqueue
+
+- VDUSE_SET_FEATURES: Set virtio features supported by the driver
+
+- VDUSE_GET_FEATURES: Get virtio features supported by the device
+
+- VDUSE_SET_STATUS: Set the device status
+
+- VDUSE_GET_STATUS: Get the device status
+
+- VDUSE_SET_CONFIG: Write to device specific configuration space
+
+- VDUSE_GET_CONFIG: Read from device specific configuration space
+
+- VDUSE_UPDATE_IOTLB: Notify userspace to update the memory mapping in device IOTLB
+
+Those control messages are mostly based on the vdpa_config_ops in
+include/linux/vdpa.h which defines a unified interface to control
+different types of vdpa device. Userspace needs to read()/write()
+on /dev/vduse/$NAME to receive/reply those control messages
+from/to VDUSE kernel module as follows:
+
+.. code-block:: c
+
+	static int vduse_message_handler(int dev_fd)
+	{
+		int len;
+		struct vduse_dev_request req;
+		struct vduse_dev_response resp;
+
+		len = read(dev_fd, &req, sizeof(req));
+		if (len != sizeof(req))
+			return -1;
+
+		resp.request_id = req.request_id;
+
+		switch (req.type) {
+
+		/* handle different types of message */
+
+		}
+
+		len = write(dev_fd, &resp, sizeof(resp));
+		if (len != sizeof(resp))
+			return -1;
+
+		return 0;
+	}
+
+In the data path, vDPA device's iova regions will be mapped into userspace
+with the help of VDUSE_IOTLB_GET_FD ioctl on /dev/vduse/$NAME:
+
+- VDUSE_IOTLB_GET_FD: get the file descriptor to the first overlapped iova region.
+  Userspace can access this iova region by passing fd and corresponding size, offset,
+  perm to mmap(). For example:
+
+.. code-block:: c
+
+	static int perm_to_prot(uint8_t perm)
+	{
+		int prot = 0;
+
+		switch (perm) {
+		case VDUSE_ACCESS_WO:
+			prot |= PROT_WRITE;
+			break;
+		case VDUSE_ACCESS_RO:
+			prot |= PROT_READ;
+			break;
+		case VDUSE_ACCESS_RW:
+			prot |= PROT_READ | PROT_WRITE;
+			break;
+		}
+
+		return prot;
+	}
+
+	static void *iova_to_va(int dev_fd, uint64_t iova, uint64_t *len)
+	{
+		int fd;
+		void *addr;
+		size_t size;
+		struct vduse_iotlb_entry entry;
+
+		entry.start = iova;
+		entry.last = iova + 1;
+		fd = ioctl(dev_fd, VDUSE_IOTLB_GET_FD, &entry);
+		if (fd < 0)
+			return NULL;
+
+		size = entry.last - entry.start + 1;
+		*len = entry.last - iova + 1;
+		addr = mmap(0, size, perm_to_prot(entry.perm), MAP_SHARED,
+			    fd, entry.offset);
+		close(fd);
+		if (addr == MAP_FAILED)
+			return NULL;
+
+		/* do something to cache this iova region */
+
+		return addr + iova - entry.start;
+	}
+
+Besides, the following ioctls on /dev/vduse/$NAME are provided to support
+interrupt injection and setting up eventfd for virtqueue kicks:
+
+- VDUSE_VQ_SETUP_KICKFD: set the kickfd for virtqueue, this eventfd is used
+  by VDUSE kernel module to notify userspace to consume the vring.
+
+- VDUSE_INJECT_VQ_IRQ: inject an interrupt for specific virtqueue
+
+- VDUSE_INJECT_CONFIG_IRQ: inject a config interrupt
+
+MMU-based IOMMU Driver
+======================
+
+VDUSE framework implements an MMU-based on-chip IOMMU driver to support
+mapping the kernel DMA buffer into the userspace iova region dynamically.
+This is mainly designed for virtio-vdpa case (kernel virtio drivers).
+
+The basic idea behind this driver is treating MMU (VA->PA) as IOMMU (IOVA->PA).
+The driver will set up MMU mapping instead of IOMMU mapping for the DMA transfer
+so that the userspace process is able to use its virtual address to access
+the DMA buffer in kernel.
+
+And to avoid security issue, a bounce-buffering mechanism is introduced to
+prevent userspace accessing the original buffer directly which may contain other
+kernel data. During the mapping, unmapping, the driver will copy the data from
+the original buffer to the bounce buffer and back, depending on the direction of
+the transfer. And the bounce-buffer addresses will be mapped into the user address
+space instead of the original one.
-- 
2.11.0