Re: [PATCH RFC net-next v1 1/5] net: add devmem TCP TX documentation

[Stanislav Fomichev <stfomichev@gmail.com>]

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.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 +235,138 @@ 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)