[PATCH v13 13/13] Documentation: Add documentation for VDUSE

Xie Yongji
2021-08-31T10:39:49Z

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>
Acked-by: Jason Wang <jasowang@redhat.com>
---
Documentation/userspace-api/index.rst | 1 +
Documentation/userspace-api/vduse.rst | 233 ++++++++++++++++++++++++++++++++++
2 files changed, 234 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..42ef59ea5314
--- /dev/null
+++ b/Documentation/userspace-api/vduse.rst
@@ -0,0 +1,233 @@
+==================================
+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.
+
+- VDUSE_UPDATE_IOTLB: Notify userspace to update the memory mapping for specified
+ IOVA range, userspace should firstly remove the old mapping, then setup the new
+ mapping via the VDUSE_IOTLB_GET_FD ioctl.
+
+After DRIVER_OK status bit is set via the VDUSE_SET_STATUS message, userspace is
+able to start the dataplane processing as follows:
+
+1. Get the specified virtqueue's information with the VDUSE_VQ_GET_INFO ioctl,
+ including the size, the IOVAs of descriptor table, available ring and used ring,
+ the state and the ready status.
+
+2. Pass the above IOVAs to the VDUSE_IOTLB_GET_FD ioctl so that those IOVA regions
+ can be mapped into userspace. Some sample codes is shown below:
+
+.. 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;
+
+ /*
+ * Find the first IOVA region that overlaps with the specified
+ * range [start, last] and return the corresponding file descriptor.
+ */
+ 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;
+
+ /*
+ * Using some data structures such as linked list to store
+ * the iotlb mapping. The munmap(2) should be called for the
+ * cached mapping when the corresponding VDUSE_UPDATE_IOTLB
+ * message is received or the device is reset.
+ */
+
+ return addr + iova - entry.start;
+ }
+
+3. Setup the kick eventfd for the specified virtqueues with the VDUSE_VQ_SETUP_KICKFD
+ ioctl. The kick eventfd is used by VDUSE kernel module to notify userspace to
+ consume the available ring. This is optional since userspace can choose to poll the
+ available ring instead.
+
+4. Listen to the kick eventfd (optional) and consume the available ring. The buffer
+ described by the descriptors in the descriptor table should be also mapped into
+ userspace via the VDUSE_IOTLB_GET_FD ioctl before accessing.
+
+5. Inject an interrupt for specific virtqueue with the VDUSE_INJECT_VQ_IRQ ioctl
+ after the used ring is filled.
+
+For more details on the uAPI, please see include/uapi/linux/vduse.h.
--
2.11.0