On Thu, Feb 27, 2025 at 04:12:05AM +0000, Mina Almasry wrote: > diff --git a/Documentation/networking/devmem.rst b/Documentation/networking/devmem.rst > index d95363645331..1c476522d6f5 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@xxxxxxxxxx/ > > > -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 > ======================== > Looks good, thanks! Reviewed-by: Bagas Sanjaya <bagasdotme@xxxxxxxxx> -- An old man doll... just what I always wanted! - Clara
Attachment:
signature.asc
Description: PGP signature
