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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Donald Hunter , Jonathan Corbet , Andrew Lunn , David Ahern , Stefan Hajnoczi , Stefano Garzarella , "Michael S. Tsirkin" , Jason Wang , Xuan Zhuo , Eugenio =?utf-8?B?UMOpcmV6?= , Shuah Khan , sdf@fomichev.me, asml.silence@gmail.com, dw@davidwei.uk, Jamal Hadi Salim , Victor Nogueira , Pedro Tammela Subject: Re: [PATCH RFC net-next v2 1/6] net: add devmem TCP TX documentation Message-ID: References: <20250130211539.428952-1-almasrymina@google.com> <20250130211539.428952-2-almasrymina@google.com> Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20250130211539.428952-2-almasrymina@google.com> On 01/30, Mina Almasry wrote: > Add documentation outlining the usage and details of the devmem TCP TX > API. > > Signed-off-by: Mina Almasry > > --- > > 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.com/ > > > -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?