1 .. SPDX-License-Identifier: GPL-2.0
2
3 =================
4 Device Memory TCP
5 =================
6
7
8 Intro
9 =====
10
11 Device memory TCP (devmem TCP) enables receiving data directly into device
12 memory (dmabuf). The feature is currently implemented for TCP sockets.
13
14
15 Opportunity
16 -----------
17
18 A large number of data transfers have device memory as the source and/or
19 destination. Accelerators drastically increased the prevalence of such
20 transfers. Some examples include:
21
22 - Distributed training, where ML accelerators, such as GPUs on different hosts,
23 exchange data.
24
25 - Distributed raw block storage applications transfer large amounts of data with
26 remote SSDs. Much of this data does not require host processing.
27
28 Typically the Device-to-Device data transfers in the network are implemented as
29 the following low-level operations: Device-to-Host copy, Host-to-Host network
30 transfer, and Host-to-Device copy.
31
32 The flow involving host copies is suboptimal, especially for bulk data transfers,
33 and can put significant strains on system resources such as host memory
34 bandwidth and PCIe bandwidth.
35
36 Devmem TCP optimizes this use case by implementing socket APIs that enable
37 the user to receive incoming network packets directly into device memory.
38
39 Packet payloads go directly from the NIC to device memory.
40
41 Packet headers go to host memory and are processed by the TCP/IP stack
42 normally. The NIC must support header split to achieve this.
43
44 Advantages:
45
46 - Alleviate host memory bandwidth pressure, compared to existing
47 network-transfer + device-copy semantics.
48
49 - Alleviate PCIe bandwidth pressure, by limiting data transfer to the lowest
50 level of the PCIe tree, compared to the traditional path which sends data
51 through the root complex.
52
53
54 More Info
55 ---------
56
57 slides, video
58 https://netdevconf.org/0x17/sessions/talk/device-memory-tcp.html
59
60 patchset
61 [PATCH net-next v24 00/13] Device Memory TCP
62 https://lore.kernel.org/netdev/20240831004313.3713467-1-almasrymina@google.com/
63
64
65 Interface
66 =========
67
68
69 Example
70 -------
71
72 tools/testing/selftests/net/ncdevmem.c:do_server shows an example of setting up
73 the RX path of this API.
74
75
76 NIC Setup
77 ---------
78
79 Header split, flow steering, & RSS are required features for devmem TCP.
80
81 Header split is used to split incoming packets into a header buffer in host
82 memory, and a payload buffer in device memory.
83
84 Flow steering & RSS are used to ensure that only flows targeting devmem land on
85 an RX queue bound to devmem.
86
87 Enable header split & flow steering::
88
89 # enable header split
90 ethtool -G eth1 tcp-data-split on
91
92
93 # enable flow steering
94 ethtool -K eth1 ntuple on
95
96 Configure RSS to steer all traffic away from the target RX queue (queue 15 in
97 this example)::
98
99 ethtool --set-rxfh-indir eth1 equal 15
100
101
102 The user must bind a dmabuf to any number of RX queues on a given NIC using
103 the netlink API::
104
105 /* Bind dmabuf to NIC RX queue 15 */
106 struct netdev_queue *queues;
107 queues = malloc(sizeof(*queues) * 1);
108
109 queues[0]._present.type = 1;
110 queues[0]._present.idx = 1;
111 queues[0].type = NETDEV_RX_QUEUE_TYPE_RX;
112 queues[0].idx = 15;
113
114 *ys = ynl_sock_create(&ynl_netdev_family, &yerr);
115
116 req = netdev_bind_rx_req_alloc();
117 netdev_bind_rx_req_set_ifindex(req, 1 /* ifindex */);
118 netdev_bind_rx_req_set_dmabuf_fd(req, dmabuf_fd);
119 __netdev_bind_rx_req_set_queues(req, queues, n_queue_index);
120
121 rsp = netdev_bind_rx(*ys, req);
122
123 dmabuf_id = rsp->dmabuf_id;
124
125
126 The netlink API returns a dmabuf_id: a unique ID that refers to this dmabuf
127 that has been bound.
128
129 The user can unbind the dmabuf from the netdevice by closing the netlink socket
130 that established the binding. We do this so that the binding is automatically
131 unbound even if the userspace process crashes.
132
133 Note that any reasonably well-behaved dmabuf from any exporter should work with
134 devmem TCP, even if the dmabuf is not actually backed by devmem. An example of
135 this is udmabuf, which wraps user memory (non-devmem) in a dmabuf.
136
137
138 Socket Setup
139 ------------
140
141 The socket must be flow steered to the dmabuf bound RX queue::
142
143 ethtool -N eth1 flow-type tcp4 ... queue 15
144
145
146 Receiving data
147 --------------
148
149 The user application must signal to the kernel that it is capable of receiving
150 devmem data by passing the MSG_SOCK_DEVMEM flag to recvmsg::
151
152 ret = recvmsg(fd, &msg, MSG_SOCK_DEVMEM);
153
154 Applications that do not specify the MSG_SOCK_DEVMEM flag will receive an EFAULT
155 on devmem data.
156
157 Devmem data is received directly into the dmabuf bound to the NIC in 'NIC
158 Setup', and the kernel signals such to the user via the SCM_DEVMEM_* cmsgs::
159
160 for (cm = CMSG_FIRSTHDR(&msg); cm; cm = CMSG_NXTHDR(&msg, cm)) {
161 if (cm->cmsg_level != SOL_SOCKET ||
162 (cm->cmsg_type != SCM_DEVMEM_DMABUF &&
163 cm->cmsg_type != SCM_DEVMEM_LINEAR))
164 continue;
165
166 dmabuf_cmsg = (struct dmabuf_cmsg *)CMSG_DATA(cm);
167
168 if (cm->cmsg_type == SCM_DEVMEM_DMABUF) {
169 /* Frag landed in dmabuf.
170 *
171 * dmabuf_cmsg->dmabuf_id is the dmabuf the
172 * frag landed on.
173 *
174 * dmabuf_cmsg->frag_offset is the offset into
175 * the dmabuf where the frag starts.
176 *
177 * dmabuf_cmsg->frag_size is the size of the
178 * frag.
179 *
180 * dmabuf_cmsg->frag_token is a token used to
181 * refer to this frag for later freeing.
182 */
183
184 struct dmabuf_token token;
185 token.token_start = dmabuf_cmsg->frag_token;
186 token.token_count = 1;
187 continue;
188 }
189
190 if (cm->cmsg_type == SCM_DEVMEM_LINEAR)
191 /* Frag landed in linear buffer.
192 *
193 * dmabuf_cmsg->frag_size is the size of the
194 * frag.
195 */
196 continue;
197
198 }
199
200 Applications may receive 2 cmsgs:
201
202 - SCM_DEVMEM_DMABUF: this indicates the fragment landed in the dmabuf indicated
203 by dmabuf_id.
204
205 - SCM_DEVMEM_LINEAR: this indicates the fragment landed in the linear buffer.
206 This typically happens when the NIC is unable to split the packet at the
207 header boundary, such that part (or all) of the payload landed in host
208 memory.
209
210 Applications may receive no SO_DEVMEM_* cmsgs. That indicates non-devmem,
211 regular TCP data that landed on an RX queue not bound to a dmabuf.
212
213
214 Freeing frags
215 -------------
216
217 Frags received via SCM_DEVMEM_DMABUF are pinned by the kernel while the user
218 processes the frag. The user must return the frag to the kernel via
219 SO_DEVMEM_DONTNEED::
220
221 ret = setsockopt(client_fd, SOL_SOCKET, SO_DEVMEM_DONTNEED, &token,
222 sizeof(token));
223
224 The user must ensure the tokens are returned to the kernel in a timely manner.
225 Failure to do so will exhaust the limited dmabuf that is bound to the RX queue
226 and will lead to packet drops.
227
228
229 Implementation & Caveats
230 ========================
231
232 Unreadable skbs
233 ---------------
234
235 Devmem payloads are inaccessible to the kernel processing the packets. This
236 results in a few quirks for payloads of devmem skbs:
237
238 - Loopback is not functional. Loopback relies on copying the payload, which is
239 not possible with devmem skbs.
240
241 - Software checksum calculation fails.
242
243 - TCP Dump and bpf can't access devmem packet payloads.
244
245
246 Testing
247 =======
248
249 More realistic example code can be found in the kernel source under
250 ``tools/testing/selftests/net/ncdevmem.c``
251
252 ncdevmem is a devmem TCP netcat. It works very similarly to netcat, but
253 receives data directly into a udmabuf.
254
255 To run ncdevmem, you need to run it on a server on the machine under test, and
256 you need to run netcat on a peer to provide the TX data.
257
258 ncdevmem has a validation mode as well that expects a repeating pattern of
259 incoming data and validates it as such. For example, you can launch
260 ncdevmem on the server by::
261
262 ncdevmem -s <server IP> -c <client IP> -f eth1 -d 3 -n 0000:06:00.0 -l \
263 -p 5201 -v 7
264
265 On client side, use regular netcat to send TX data to ncdevmem process
266 on the server::
267
268 yes $(echo -e \\x01\\x02\\x03\\x04\\x05\\x06) | \
269 tr \\n \\0 | head -c 5G | nc <server IP> 5201 -p 5201
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