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@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

Looks good, thanks!

Reviewed-by: Bagas Sanjaya bagasdotme@gmail.com