linux/Documentation/userspace-api/vduse.rst
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   1==================================
   2VDUSE - "vDPA Device in Userspace"
   3==================================
   4
   5vDPA (virtio data path acceleration) device is a device that uses a
   6datapath which complies with the virtio specifications with vendor
   7specific control path. vDPA devices can be both physically located on
   8the hardware or emulated by software. VDUSE is a framework that makes it
   9possible to implement software-emulated vDPA devices in userspace. And
  10to make the device emulation more secure, the emulated vDPA device's
  11control path is handled in the kernel and only the data path is
  12implemented in the userspace.
  13
  14Note that only virtio block device is supported by VDUSE framework now,
  15which can reduce security risks when the userspace process that implements
  16the data path is run by an unprivileged user. The support for other device
  17types can be added after the security issue of corresponding device driver
  18is clarified or fixed in the future.
  19
  20Create/Destroy VDUSE devices
  21----------------------------
  22
  23VDUSE devices are created as follows:
  24
  251. Create a new VDUSE instance with ioctl(VDUSE_CREATE_DEV) on
  26   /dev/vduse/control.
  27
  282. Setup each virtqueue with ioctl(VDUSE_VQ_SETUP) on /dev/vduse/$NAME.
  29
  303. Begin processing VDUSE messages from /dev/vduse/$NAME. The first
  31   messages will arrive while attaching the VDUSE instance to vDPA bus.
  32
  334. Send the VDPA_CMD_DEV_NEW netlink message to attach the VDUSE
  34   instance to vDPA bus.
  35
  36VDUSE devices are destroyed as follows:
  37
  381. Send the VDPA_CMD_DEV_DEL netlink message to detach the VDUSE
  39   instance from vDPA bus.
  40
  412. Close the file descriptor referring to /dev/vduse/$NAME.
  42
  433. Destroy the VDUSE instance with ioctl(VDUSE_DESTROY_DEV) on
  44   /dev/vduse/control.
  45
  46The netlink messages can be sent via vdpa tool in iproute2 or use the
  47below sample codes:
  48
  49.. code-block:: c
  50
  51        static int netlink_add_vduse(const char *name, enum vdpa_command cmd)
  52        {
  53                struct nl_sock *nlsock;
  54                struct nl_msg *msg;
  55                int famid;
  56
  57                nlsock = nl_socket_alloc();
  58                if (!nlsock)
  59                        return -ENOMEM;
  60
  61                if (genl_connect(nlsock))
  62                        goto free_sock;
  63
  64                famid = genl_ctrl_resolve(nlsock, VDPA_GENL_NAME);
  65                if (famid < 0)
  66                        goto close_sock;
  67
  68                msg = nlmsg_alloc();
  69                if (!msg)
  70                        goto close_sock;
  71
  72                if (!genlmsg_put(msg, NL_AUTO_PORT, NL_AUTO_SEQ, famid, 0, 0, cmd, 0))
  73                        goto nla_put_failure;
  74
  75                NLA_PUT_STRING(msg, VDPA_ATTR_DEV_NAME, name);
  76                if (cmd == VDPA_CMD_DEV_NEW)
  77                        NLA_PUT_STRING(msg, VDPA_ATTR_MGMTDEV_DEV_NAME, "vduse");
  78
  79                if (nl_send_sync(nlsock, msg))
  80                        goto close_sock;
  81
  82                nl_close(nlsock);
  83                nl_socket_free(nlsock);
  84
  85                return 0;
  86        nla_put_failure:
  87                nlmsg_free(msg);
  88        close_sock:
  89                nl_close(nlsock);
  90        free_sock:
  91                nl_socket_free(nlsock);
  92                return -1;
  93        }
  94
  95How VDUSE works
  96---------------
  97
  98As mentioned above, a VDUSE device is created by ioctl(VDUSE_CREATE_DEV) on
  99/dev/vduse/control. With this ioctl, userspace can specify some basic configuration
 100such as device name (uniquely identify a VDUSE device), virtio features, virtio
 101configuration space, the number of virtqueues and so on for this emulated device.
 102Then a char device interface (/dev/vduse/$NAME) is exported to userspace for device
 103emulation. Userspace can use the VDUSE_VQ_SETUP ioctl on /dev/vduse/$NAME to
 104add per-virtqueue configuration such as the max size of virtqueue to the device.
 105
 106After the initialization, the VDUSE device can be attached to vDPA bus via
 107the VDPA_CMD_DEV_NEW netlink message. Userspace needs to read()/write() on
 108/dev/vduse/$NAME to receive/reply some control messages from/to VDUSE kernel
 109module as follows:
 110
 111.. code-block:: c
 112
 113        static int vduse_message_handler(int dev_fd)
 114        {
 115                int len;
 116                struct vduse_dev_request req;
 117                struct vduse_dev_response resp;
 118
 119                len = read(dev_fd, &req, sizeof(req));
 120                if (len != sizeof(req))
 121                        return -1;
 122
 123                resp.request_id = req.request_id;
 124
 125                switch (req.type) {
 126
 127                /* handle different types of messages */
 128
 129                }
 130
 131                len = write(dev_fd, &resp, sizeof(resp));
 132                if (len != sizeof(resp))
 133                        return -1;
 134
 135                return 0;
 136        }
 137
 138There are now three types of messages introduced by VDUSE framework:
 139
 140- VDUSE_GET_VQ_STATE: Get the state for virtqueue, userspace should return
 141  avail index for split virtqueue or the device/driver ring wrap counters and
 142  the avail and used index for packed virtqueue.
 143
 144- VDUSE_SET_STATUS: Set the device status, userspace should follow
 145  the virtio spec: https://docs.oasis-open.org/virtio/virtio/v1.1/virtio-v1.1.html
 146  to process this message. For example, fail to set the FEATURES_OK device
 147  status bit if the device can not accept the negotiated virtio features
 148  get from the VDUSE_DEV_GET_FEATURES ioctl.
 149
 150- VDUSE_UPDATE_IOTLB: Notify userspace to update the memory mapping for specified
 151  IOVA range, userspace should firstly remove the old mapping, then setup the new
 152  mapping via the VDUSE_IOTLB_GET_FD ioctl.
 153
 154After DRIVER_OK status bit is set via the VDUSE_SET_STATUS message, userspace is
 155able to start the dataplane processing as follows:
 156
 1571. Get the specified virtqueue's information with the VDUSE_VQ_GET_INFO ioctl,
 158   including the size, the IOVAs of descriptor table, available ring and used ring,
 159   the state and the ready status.
 160
 1612. Pass the above IOVAs to the VDUSE_IOTLB_GET_FD ioctl so that those IOVA regions
 162   can be mapped into userspace. Some sample codes is shown below:
 163
 164.. code-block:: c
 165
 166        static int perm_to_prot(uint8_t perm)
 167        {
 168                int prot = 0;
 169
 170                switch (perm) {
 171                case VDUSE_ACCESS_WO:
 172                        prot |= PROT_WRITE;
 173                        break;
 174                case VDUSE_ACCESS_RO:
 175                        prot |= PROT_READ;
 176                        break;
 177                case VDUSE_ACCESS_RW:
 178                        prot |= PROT_READ | PROT_WRITE;
 179                        break;
 180                }
 181
 182                return prot;
 183        }
 184
 185        static void *iova_to_va(int dev_fd, uint64_t iova, uint64_t *len)
 186        {
 187                int fd;
 188                void *addr;
 189                size_t size;
 190                struct vduse_iotlb_entry entry;
 191
 192                entry.start = iova;
 193                entry.last = iova;
 194
 195                /*
 196                 * Find the first IOVA region that overlaps with the specified
 197                 * range [start, last] and return the corresponding file descriptor.
 198                 */
 199                fd = ioctl(dev_fd, VDUSE_IOTLB_GET_FD, &entry);
 200                if (fd < 0)
 201                        return NULL;
 202
 203                size = entry.last - entry.start + 1;
 204                *len = entry.last - iova + 1;
 205                addr = mmap(0, size, perm_to_prot(entry.perm), MAP_SHARED,
 206                            fd, entry.offset);
 207                close(fd);
 208                if (addr == MAP_FAILED)
 209                        return NULL;
 210
 211                /*
 212                 * Using some data structures such as linked list to store
 213                 * the iotlb mapping. The munmap(2) should be called for the
 214                 * cached mapping when the corresponding VDUSE_UPDATE_IOTLB
 215                 * message is received or the device is reset.
 216                 */
 217
 218                return addr + iova - entry.start;
 219        }
 220
 2213. Setup the kick eventfd for the specified virtqueues with the VDUSE_VQ_SETUP_KICKFD
 222   ioctl. The kick eventfd is used by VDUSE kernel module to notify userspace to
 223   consume the available ring. This is optional since userspace can choose to poll the
 224   available ring instead.
 225
 2264. Listen to the kick eventfd (optional) and consume the available ring. The buffer
 227   described by the descriptors in the descriptor table should be also mapped into
 228   userspace via the VDUSE_IOTLB_GET_FD ioctl before accessing.
 229
 2305. Inject an interrupt for specific virtqueue with the VDUSE_INJECT_VQ_IRQ ioctl
 231   after the used ring is filled.
 232
 233For more details on the uAPI, please see include/uapi/linux/vduse.h.
 234