On 12/21, Mina Almasry wrote: > Add documentation outlining the usage and details of the devmem TCP TX > API. > > Signed-off-by: Mina Almasry <almasrymina@xxxxxxxxxx> > --- > Documentation/networking/devmem.rst | 140 +++++++++++++++++++++++++++- > 1 file changed, 136 insertions(+), 4 deletions(-) > > diff --git a/Documentation/networking/devmem.rst b/Documentation/networking/devmem.rst > index d95363645331..9be01cd96ee2 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 +235,138 @@ 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 with iov_base set to NULL and iov_len set to the > +number of bytes to be sent from the dmabuf. Should we verify that iov_base is NULL in the kernel? But also, alternatively, why not go with iov_base == offset? This way we can support several offsets in a single message, just like regular sendmsg with host memory. Any reason to not do that? > +The user passes the dma-buf id via the dmabuf_tx_cmsg.dmabuf_id, and passes the > +offset into the dmabuf from where to start sending using the > +dmabuf_tx_cmsg.dmabuf_offset field:: > + [...] > + char ctrl_data[CMSG_SPACE(sizeof(struct dmabuf_tx_cmsg))]; > + struct dmabuf_tx_cmsg ddmabuf; > + struct msghdr msg = {}; > + struct cmsghdr *cmsg; > + uint64_t off = 100; > + struct iovec iov; > + > + iov.iov_base = NULL; > + iov.iov_len = line_size; nit: indent seems to be different (tabs vs spaces)
