On 01/30, Mina Almasry wrote:
Add documentation outlining the usage and details of the devmem TCP TX API.
Signed-off-by: Mina Almasry almasrymina@google.com
v2:
- Update documentation for iov_base is the dmabuf offset (Stan)
Documentation/networking/devmem.rst | 144 +++++++++++++++++++++++++++- 1 file changed, 140 insertions(+), 4 deletions(-)
diff --git a/Documentation/networking/devmem.rst b/Documentation/networking/devmem.rst index d95363645331..8166fe09da13 100644 --- a/Documentation/networking/devmem.rst +++ b/Documentation/networking/devmem.rst @@ -62,15 +62,15 @@ More Info https://lore.kernel.org/netdev/20240831004313.3713467-1-almasrymina@google.c...
-Interface
+RX Interface +============ Example
-tools/testing/selftests/net/ncdevmem.c:do_server shows an example of setting up -the RX path of this API. +./tools/testing/selftests/drivers/net/hw/ncdevmem:do_server shows an example of +setting up the RX path of this API. NIC Setup @@ -235,6 +235,142 @@ can be less than the tokens provided by the user in case of: (a) an internal kernel leak bug. (b) the user passed more than 1024 frags. +TX Interface +============
+Example +-------
+./tools/testing/selftests/drivers/net/hw/ncdevmem:do_client shows an example of +setting up the TX path of this API.
+NIC Setup +---------
+The user must bind a TX dmabuf to a given NIC using the netlink API::
struct netdev_bind_tx_req *req = NULL;struct netdev_bind_tx_rsp *rsp = NULL;struct ynl_error yerr;*ys = ynl_sock_create(&ynl_netdev_family, &yerr);req = netdev_bind_tx_req_alloc();netdev_bind_tx_req_set_ifindex(req, ifindex);netdev_bind_tx_req_set_fd(req, dmabuf_fd);rsp = netdev_bind_tx(*ys, req);tx_dmabuf_id = rsp->id;+The netlink API returns a dmabuf_id: a unique ID that refers to this dmabuf +that has been bound.
+The user can unbind the dmabuf from the netdevice by closing the netlink socket +that established the binding. We do this so that the binding is automatically +unbound even if the userspace process crashes.
+Note that any reasonably well-behaved dmabuf from any exporter should work with +devmem TCP, even if the dmabuf is not actually backed by devmem. An example of +this is udmabuf, which wraps user memory (non-devmem) in a dmabuf.
+Socket Setup +------------
+The user application must use MSG_ZEROCOPY flag when sending devmem TCP. Devmem +cannot be copied by the kernel, so the semantics of the devmem TX are similar +to the semantics of MSG_ZEROCOPY.
- ret = setsockopt(socket_fd, SOL_SOCKET, SO_ZEROCOPY, &opt, sizeof(opt));
+Sending data +--------------
+Devmem data is sent using the SCM_DEVMEM_DMABUF cmsg.
+The user should create a msghdr where,
+iov_base is set to the offset into the dmabuf to start sending from. +iov_len is set to the number of bytes to be sent from the dmabuf.
+The user passes the dma-buf id to send from via the dmabuf_tx_cmsg.dmabuf_id.
+The example below sends 1024 bytes from offset 100 into the dmabuf, and 2048 +from offset 2000 into the dmabuf. The dmabuf to send from is tx_dmabuf_id::
char ctrl_data[CMSG_SPACE(sizeof(struct dmabuf_tx_cmsg))];struct dmabuf_tx_cmsg ddmabuf;struct msghdr msg = {};struct cmsghdr *cmsg;struct iovec iov[2];iov[0].iov_base = (void*)100;iov[0].iov_len = 1024;iov[1].iov_base = (void*)2000;iov[1].iov_len = 2048;msg.msg_iov = iov;msg.msg_iovlen = 2;msg.msg_control = ctrl_data;msg.msg_controllen = sizeof(ctrl_data);cmsg = CMSG_FIRSTHDR(&msg);cmsg->cmsg_level = SOL_SOCKET;cmsg->cmsg_type = SCM_DEVMEM_DMABUF;cmsg->cmsg_len = CMSG_LEN(sizeof(struct dmabuf_tx_cmsg));ddmabuf.dmabuf_id = tx_dmabuf_id;*((struct dmabuf_tx_cmsg *)CMSG_DATA(cmsg)) = ddmabuf;
[..]
sendmsg(socket_fd, &msg, MSG_ZEROCOPY);
Not super important, but any reason not to use MSG_SOCK_DEVMEM as a flag? We already use it for recvmsg, seems logical to mirror the same flag on the transmit side?