Subject: Re: [PATCH RFC net-next v2 1/6] net: add devmem TCP TX documentation

From: Stanislav Fomichev <[email-masked]>
Date: Thu, 30 Jan 2025 14:59:10 -0800

On 01/30, Mina Almasry wrote:
> Add documentation outlining the usage and details of the devmem TCP TX
> API.
> 
> Signed-off-by: Mina Almasry <[email-masked]>
> 
> ---
> 
> 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/[email-masked]/
>  
>  
> -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?