Add documentation outlining the usage and details of the devmem TCP TX API.
Signed-off-by: Mina Almasry almasrymina@google.com Acked-by: Stanislav Fomichev sdf@fomichev.me
---
v5: - Address comments from Stan and Bagas
v4: - Mention SO_BINDTODEVICE is recommended (me/Pavel).
v2: - Update documentation for iov_base is the dmabuf offset (Stan)
--- Documentation/networking/devmem.rst | 150 +++++++++++++++++++++++++++- 1 file changed, 146 insertions(+), 4 deletions(-)
diff --git a/Documentation/networking/devmem.rst b/Documentation/networking/devmem.rst index eb678ca45496..a6cd7236bfbd 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 +235,148 @@ 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:: + + setsockopt(socket_fd, SOL_SOCKET, SO_ZEROCOPY, &opt, sizeof(opt)); + +It is also recommended that the user binds the TX socket to the same interface +the dma-buf has been bound to via SO_BINDTODEVICE:: + + setsockopt(socket_fd, SOL_SOCKET, SO_BINDTODEVICE, ifname, strlen(ifname) + 1); + + +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); + + +Reusing TX dmabufs +------------------ + +Similar to MSG_ZEROCOPY with regular memory, the user should not modify the +contents of the dma-buf while a send operation is in progress. This is because +the kernel does not keep a copy of the dmabuf contents. Instead, the kernel +will pin and send data from the buffer available to the userspace. + +Just as in MSG_ZEROCOPY, the kernel notifies the userspace of send completions +using MSG_ERRQUEUE:: + + int64_t tstop = gettimeofday_ms() + waittime_ms; + char control[CMSG_SPACE(100)] = {}; + struct sock_extended_err *serr; + struct msghdr msg = {}; + struct cmsghdr *cm; + int retries = 10; + __u32 hi, lo; + + msg.msg_control = control; + msg.msg_controllen = sizeof(control); + + while (gettimeofday_ms() < tstop) { + if (!do_poll(fd)) continue; + + ret = recvmsg(fd, &msg, MSG_ERRQUEUE); + + for (cm = CMSG_FIRSTHDR(&msg); cm; cm = CMSG_NXTHDR(&msg, cm)) { + serr = (void *)CMSG_DATA(cm); + + hi = serr->ee_data; + lo = serr->ee_info; + + fprintf(stdout, "tx complete [%d,%d]\n", lo, hi); + } + } + +After the associated sendmsg has been completed, the dmabuf can be reused by +the userspace. + + Implementation & Caveats ========================