net: add devmem TCP documentation

.. SPDX-License-Identifier: GPL-2.0

=================

Device Memory TCP

=================

Intro

=====

Device memory TCP (devmem TCP) enables receiving data directly into device

memory (dmabuf). The feature is currently implemented for TCP sockets.

Opportunity

-----------

A large number of data transfers have device memory as the source and/or

destination. Accelerators drastically increased the prevalence of such

transfers.  Some examples include:

- Distributed training, where ML accelerators, such as GPUs on different hosts,

  exchange data.

- Distributed raw block storage applications transfer large amounts of data with

  remote SSDs. Much of this data does not require host processing.

Typically the Device-to-Device data transfers in the network are implemented as

the following low-level operations: Device-to-Host copy, Host-to-Host network

transfer, and Host-to-Device copy.

The flow involving host copies is suboptimal, especially for bulk data transfers,

and can put significant strains on system resources such as host memory

bandwidth and PCIe bandwidth.

Devmem TCP optimizes this use case by implementing socket APIs that enable

the user to receive incoming network packets directly into device memory.

Packet payloads go directly from the NIC to device memory.

Packet headers go to host memory and are processed by the TCP/IP stack

normally. The NIC must support header split to achieve this.

Advantages:

- Alleviate host memory bandwidth pressure, compared to existing

  network-transfer + device-copy semantics.

- Alleviate PCIe bandwidth pressure, by limiting data transfer to the lowest

  level of the PCIe tree, compared to the traditional path which sends data

  through the root complex.

More Info

---------

  slides, video

    https://netdevconf.org/0x17/sessions/talk/device-memory-tcp.html

  patchset

    [PATCH net-next v24 00/13] Device Memory TCP

    https://lore.kernel.org/netdev/20240831004313.3713467-1-almasrymina@google.com/

Interface

=========

Example

-------

tools/testing/selftests/net/ncdevmem.c:do_server shows an example of setting up

the RX path of this API.

NIC Setup

---------

Header split, flow steering, & RSS are required features for devmem TCP.

Header split is used to split incoming packets into a header buffer in host

memory, and a payload buffer in device memory.

Flow steering & RSS are used to ensure that only flows targeting devmem land on

an RX queue bound to devmem.

Enable header split & flow steering::

	# enable header split

	ethtool -G eth1 tcp-data-split on

	# enable flow steering

	ethtool -K eth1 ntuple on

Configure RSS to steer all traffic away from the target RX queue (queue 15 in

this example)::

	ethtool --set-rxfh-indir eth1 equal 15

The user must bind a dmabuf to any number of RX queues on a given NIC using

the netlink API::

	/* Bind dmabuf to NIC RX queue 15 */

	struct netdev_queue *queues;

	queues = malloc(sizeof(*queues) * 1);

	queues[0]._present.type = 1;

	queues[0]._present.idx = 1;

	queues[0].type = NETDEV_RX_QUEUE_TYPE_RX;

	queues[0].idx = 15;

	*ys = ynl_sock_create(&ynl_netdev_family, &yerr);

	req = netdev_bind_rx_req_alloc();

	netdev_bind_rx_req_set_ifindex(req, 1 /* ifindex */);

	netdev_bind_rx_req_set_dmabuf_fd(req, dmabuf_fd);

	__netdev_bind_rx_req_set_queues(req, queues, n_queue_index);

	rsp = netdev_bind_rx(*ys, req);

	dmabuf_id = rsp->dmabuf_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 socket must be flow steered to the dmabuf bound RX queue::

	ethtool -N eth1 flow-type tcp4 ... queue 15

Receiving data

--------------

The user application must signal to the kernel that it is capable of receiving

devmem data by passing the MSG_SOCK_DEVMEM flag to recvmsg::

	ret = recvmsg(fd, &msg, MSG_SOCK_DEVMEM);

Applications that do not specify the MSG_SOCK_DEVMEM flag will receive an EFAULT

on devmem data.

Devmem data is received directly into the dmabuf bound to the NIC in 'NIC

Setup', and the kernel signals such to the user via the SCM_DEVMEM_* cmsgs::

		for (cm = CMSG_FIRSTHDR(&msg); cm; cm = CMSG_NXTHDR(&msg, cm)) {

			if (cm->cmsg_level != SOL_SOCKET ||

				(cm->cmsg_type != SCM_DEVMEM_DMABUF &&

				 cm->cmsg_type != SCM_DEVMEM_LINEAR))

				continue;

			dmabuf_cmsg = (struct dmabuf_cmsg *)CMSG_DATA(cm);

			if (cm->cmsg_type == SCM_DEVMEM_DMABUF) {

				/* Frag landed in dmabuf.

				 *

				 * dmabuf_cmsg->dmabuf_id is the dmabuf the

				 * frag landed on.

				 *

				 * dmabuf_cmsg->frag_offset is the offset into

				 * the dmabuf where the frag starts.

				 *

				 * dmabuf_cmsg->frag_size is the size of the

				 * frag.

				 *

				 * dmabuf_cmsg->frag_token is a token used to

				 * refer to this frag for later freeing.

				 */

				struct dmabuf_token token;

				token.token_start = dmabuf_cmsg->frag_token;

				token.token_count = 1;

				continue;

			}

			if (cm->cmsg_type == SCM_DEVMEM_LINEAR)

				/* Frag landed in linear buffer.

				 *

				 * dmabuf_cmsg->frag_size is the size of the

				 * frag.

				 */

				continue;

		}

Applications may receive 2 cmsgs:

- SCM_DEVMEM_DMABUF: this indicates the fragment landed in the dmabuf indicated

  by dmabuf_id.

- SCM_DEVMEM_LINEAR: this indicates the fragment landed in the linear buffer.

  This typically happens when the NIC is unable to split the packet at the

  header boundary, such that part (or all) of the payload landed in host

  memory.

Applications may receive no SO_DEVMEM_* cmsgs. That indicates non-devmem,

regular TCP data that landed on an RX queue not bound to a dmabuf.

Freeing frags

-------------

Frags received via SCM_DEVMEM_DMABUF are pinned by the kernel while the user

processes the frag. The user must return the frag to the kernel via

SO_DEVMEM_DONTNEED::

	ret = setsockopt(client_fd, SOL_SOCKET, SO_DEVMEM_DONTNEED, &token,

			 sizeof(token));

The user must ensure the tokens are returned to the kernel in a timely manner.

Failure to do so will exhaust the limited dmabuf that is bound to the RX queue

and will lead to packet drops.

Implementation & Caveats

========================

Unreadable skbs

---------------

Devmem payloads are inaccessible to the kernel processing the packets. This

results in a few quirks for payloads of devmem skbs:

- Loopback is not functional. Loopback relies on copying the payload, which is

  not possible with devmem skbs.

- Software checksum calculation fails.

- TCP Dump and bpf can't access devmem packet payloads.

Testing

=======

More realistic example code can be found in the kernel source under

``tools/testing/selftests/net/ncdevmem.c``

ncdevmem is a devmem TCP netcat. It works very similarly to netcat, but

receives data directly into a udmabuf.

To run ncdevmem, you need to run it on a server on the machine under test, and

you need to run netcat on a peer to provide the TX data.

ncdevmem has a validation mode as well that expects a repeating pattern of

incoming data and validates it as such. For example, you can launch

ncdevmem on the server by::

	ncdevmem -s <server IP> -c <client IP> -f eth1 -d 3 -n 0000:06:00.0 -l \

		 -p 5201 -v 7

On client side, use regular netcat to send TX data to ncdevmem process

on the server::

	yes $(echo -e \\x01\\x02\\x03\\x04\\x05\\x06) | \

		tr \\n \\0 | head -c 5G | nc <server IP> 5201 -p 5201

   cdc_mbim

   dccp

   dctcp

   devmem

   dns_resolver

   driver

   eql