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@google.com

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@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 +235,138 @@ 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)