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    Subject[PATCH v8 10/10] Documentation: Add documentation for VDUSE
    Date
    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 | 222 ++++++++++++++++++++++++++++++++++
    2 files changed, 223 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..2f9cd1a4e530
    --- /dev/null
    +++ b/Documentation/userspace-api/vduse.rst
    @@ -0,0 +1,222 @@
    +==================================
    +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 it simple, 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 is clarified or fixed in the future.
    +
    +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/$NAME. The first
    + messages will arrive while attaching the VDUSE instance to vDPA bus.
    +
    +3. Send the VDPA_CMD_DEV_NEW netlink message to attach the VDUSE
    + instance to vDPA bus.
    +
    +VDUSE devices are stopped as follows:
    +
    +1. Send the VDPA_CMD_DEV_DEL netlink message to detach the VDUSE
    + instance from vDPA bus.
    +
    +2. Close the file descriptor referring to /dev/vduse/$NAME
    +
    +3. Destroy the VDUSE instance with ioctl(VDUSE_DESTROY_DEV) on
    + /dev/vduse/control
    +
    +The netlink messages 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;
    + }
    +
    +How VDUSE works
    +---------------
    +
    +Since the emuldated vDPA device's control path is handled in the kernel,
    +a message-based communication protocol and few types of control messages
    +are introduced by VDUSE framework to make userspace be aware of the data
    +path related changes:
    +
    +- VDUSE_GET_VQ_STATE: Get the state for virtqueue from userspace
    +
    +- VDUSE_START_DATAPLANE: Notify userspace to start the dataplane
    +
    +- VDUSE_STOP_DATAPLANE: Notify userspace to stop the dataplane
    +
    +- VDUSE_UPDATE_IOTLB: Notify userspace to update the memory mapping in device IOTLB
    +
    +Userspace needs to read()/write() on /dev/vduse/$NAME to receive/reply
    +those control messages from/to VDUSE kernel module as follows:
    +
    +.. code-block:: c
    +
    + static int vduse_message_handler(int dev_fd)
    + {
    + int len;
    + struct vduse_dev_request req;
    + struct vduse_dev_response resp;
    +
    + len = read(dev_fd, &req, sizeof(req));
    + if (len != sizeof(req))
    + return -1;
    +
    + resp.request_id = req.request_id;
    +
    + switch (req.type) {
    +
    + /* handle different types of message */
    +
    + }
    +
    + if (req.flags & VDUSE_REQ_FLAGS_NO_REPLY)
    + return 0;
    +
    + len = write(dev_fd, &resp, sizeof(resp));
    + if (len != sizeof(resp))
    + return -1;
    +
    + return 0;
    + }
    +
    +After VDUSE_START_DATAPLANE messages is received, userspace should start the
    +dataplane processing with the help of some ioctls on /dev/vduse/$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;
    + }
    +
    +- VDUSE_DEV_GET_FEATURES: Get the negotiated features
    +
    +- VDUSE_DEV_UPDATE_CONFIG: Update the configuration space and inject a config interrupt
    +
    +- VDUSE_VQ_GET_INFO: Get the specified virtqueue's metadata
    +
    +- 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
    +
    +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
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