On 01/30, Mina Almasry wrote: > On Thu, Jan 30, 2025 at 2:59 PM Stanislav Fomichev <stfomichev@xxxxxxxxx> wrote: > > > > 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@xxxxxxxxxx> > > > > > > --- > > > > > > 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@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 +235,142 @@ 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? > > Only to remove redundancy, and the possible confusion that could > arise, and the extra checks needed to catch invalid input. > > With this, the user tells the kernel to send from the dmabuf by > supplying the SCM_DEVMEM_DMABUF cmsg. If we add another signal like > MSG_SOCK_DEVMEM, there is room for the user to supply the cmg but not > the flag (confusion), and the kernel needs to have the code and > overhead to check that both the flag and the cmsg are provided. SG! Having another 'if' doesn't seem like a big overhead to me (given that we already spend a ton of time copying all the cmsg stuff), but no strong preference on my side..
