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1 ## @file
2 #
3 # Technical notes for the virtio-net driver.
4 #
5 # Copyright (C) 2013, Red Hat, Inc.
6 #
7 # This program and the accompanying materials are licensed and made available
8 # under the terms and conditions of the BSD License which accompanies this
9 # distribution. The full text of the license may be found at
10 # http://opensource.org/licenses/bsd-license.php
11 #
12 # THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS, WITHOUT
13 # WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
14 #
15 ##
16
17 Disclaimer
18 ----------
19
20 All statements concerning standards and specifications are informative and not
21 normative. They are made in good faith. Corrections are most welcome on the
22 edk2-devel mailing list.
23
24 The following documents have been perused while writing the driver and this
25 document:
26 - Unified Extensible Firmware Interface Specification, Version 2.3.1, Errata C;
27 June 27, 2012
28 - Driver Writer's Guide for UEFI 2.3.1, 03/08/2012, Version 1.01;
29 - Virtio PCI Card Specification, v0.9.5 DRAFT, 2012 May 7.
30
31
32 Summary
33 -------
34
35 The VirtioNetDxe UEFI_DRIVER implements the Simple Network Protocol for
36 virtio-net devices. Higher level protocols are automatically installed on top
37 of it by the DXE Core / the ConnectController() boot service, enabling for
38 virtio-net devices eg. DHCP configuration, TCP transfers with edk2 StdLib
39 applications, and PXE booting in OVMF.
40
41
42 UEFI driver structure
43 ---------------------
44
45 A driver instance, belonging to a given virtio-net device, can be in one of
46 four states at any time. The states stack up as follows below. The state
47 transitions are labeled with the primary function (and its important callees
48 faithfully indented) that implement the transition.
49
50 | ^
51 | |
52 [DriverBinding.c] | | [DriverBinding.c]
53 VirtioNetDriverBindingStart | | VirtioNetDriverBindingStop
54 VirtioNetSnpPopulate | | VirtioNetSnpEvacuate
55 VirtioNetGetFeatures | |
56 v |
57 +-------------------------+
58 | EfiSimpleNetworkStopped |
59 +-------------------------+
60 | ^
61 [SnpStart.c] | | [SnpStop.c]
62 VirtioNetStart | | VirtioNetStop
63 | |
64 v |
65 +-------------------------+
66 | EfiSimpleNetworkStarted |
67 +-------------------------+
68 | ^
69 [SnpInitialize.c] | | [SnpShutdown.c]
70 VirtioNetInitialize | | VirtioNetShutdown
71 VirtioNetInitRing {Rx, Tx} | | VirtioNetShutdownRx [SnpSharedHelpers.c]
72 VirtioRingInit | | VirtioNetShutdownTx [SnpSharedHelpers.c]
73 VirtioNetInitTx | | VirtioRingUninit {Tx, Rx}
74 VirtioNetInitRx | |
75 v |
76 +-----------------------------+
77 | EfiSimpleNetworkInitialized |
78 +-----------------------------+
79
80 The state at the top means "nonexistent" and is hence unnamed on the diagram --
81 a driver instance actually doesn't exist at that point. The transition
82 functions out of and into that state implement the Driver Binding Protocol.
83
84 The lower three states characterize an existent driver instance and are all
85 states defined by the Simple Network Protocol. The transition functions between
86 them are member functions of the Simple Network Protocol.
87
88 Each transition function validates its expected source state and its
89 parameters. For example, VirtioNetDriverBindingStop will refuse to disconnect
90 from the controller unless it's in EfiSimpleNetworkStopped.
91
92
93 Driver instance states (Simple Network Protocol)
94 ------------------------------------------------
95
96 In the EfiSimpleNetworkStopped state, the virtio-net device is (has been)
97 re-set. No resources are allocated for networking / traffic purposes. The MAC
98 address and other device attributes have been retrieved from the device (this
99 is necessary for completing the VirtioNetDriverBindingStart transition).
100
101 The EfiSimpleNetworkStarted is completely identical to the
102 EfiSimpleNetworkStopped state for virtio-net, in the functional and
103 resource-usage sense. This state is mandated / provided by the Simple Network
104 Protocol for flexibility that the virtio-net driver doesn't exploit.
105
106 In particular, the EfiSimpleNetworkStarted state is the target of the Shutdown
107 SNP member function, and must therefore correspond to a hardware configuration
108 where "[it] is safe for another driver to initialize". (Clearly another UEFI
109 driver could not do that due to the exclusivity of the driver binding that
110 VirtioNetDriverBindingStart() installs, but a later OS driver might qualify.)
111
112 The EfiSimpleNetworkInitialized state is the live state of the virtio NIC / the
113 driver instance. Virtio and other resources required for network traffic have
114 been allocated, and the following SNP member functions are available (in
115 addition to VirtioNetShutdown which leaves the state):
116
117 - VirtioNetReceive [SnpReceive.c]: poll the virtio NIC for an Rx packet that
118 may have arrived asynchronously;
119
120 - VirtioNetTransmit [SnpTransmit.c]: queue a Tx packet for asynchronous
121 transmission (meant to be used together with VirtioNetGetStatus);
122
123 - VirtioNetGetStatus [SnpGetStatus.c]: query link status and status of pending
124 Tx packets;
125
126 - VirtioNetMcastIpToMac [SnpMcastIpToMac.c]: transform a multicast IPv4/IPv6
127 address into a multicast MAC address;
128
129 - VirtioNetReceiveFilters [SnpReceiveFilters.c]: emulate unicast / multicast /
130 broadcast filter configuration (not their actual effect -- a more liberal
131 filter setting than requested is allowed by the UEFI specification).
132
133 The following SNP member functions are not supported [SnpUnsupported.c]:
134
135 - VirtioNetReset: reinitialize the virtio NIC without shutting it down (a loop
136 from/to EfiSimpleNetworkInitialized);
137
138 - VirtioNetStationAddress: assign a new MAC address to the virtio NIC,
139
140 - VirtioNetStatistics: collect statistics,
141
142 - VirtioNetNvData: access non-volatile data on the virtio NIC.
143
144 Missing support for these functions is allowed by the UEFI specification and
145 doesn't seem to trip up higher level protocols.
146
147
148 Events and task priority levels
149 -------------------------------
150
151 The UEFI specification defines a sophisticated mechanism for asynchronous
152 events / callbacks (see "6.1 Event, Timer, and Task Priority Services" for
153 details). Such callbacks work like software interrupts, and some notion of
154 locking / masking is important to implement critical sections (atomic or
155 exclusive access to data or a device). This notion is defined as Task Priority
156 Levels.
157
158 The virtio-net driver for OVMF must concern itself with events for two reasons:
159
160 - The Simple Network Protocol provides its clients with a (non-optional) WAIT
161 type event called WaitForPacket: it allows them to check or wait for Rx
162 packets by polling or blocking on this event. (This functionality overlaps
163 with the Receive member function.) The event is available to clients starting
164 with EfiSimpleNetworkStopped (inclusive).
165
166 The virtio-net driver is informed about such client polling or blockage by
167 receiving an asynchronous callback (a software interrupt). In the callback
168 function the driver must interrogate the driver instance state, and if it is
169 EfiSimpleNetworkInitialized, access the Rx queue and see if any packets are
170 available for consumption. If so, it must signal the WaitForPacket WAIT type
171 event, waking the client.
172
173 For simplicity and safety, all parts of the virtio-net driver that access any
174 bit of the driver instance (data or device) run at the TPL_CALLBACK level.
175 This is the highest level allowed for an SNP implementation, and all code
176 protected in this manner satisfies even stricter non-blocking requirements
177 than what's documented for TPL_CALLBACK.
178
179 The task priority level for the WaitForPacket callback too is set by the
180 driver, the choice is TPL_CALLBACK again. This in effect serializes the
181 WaitForPacket callback (VirtioNetIsPacketAvailable [Events.c]) with "normal"
182 parts of the driver.
183
184 - According to the Driver Writer's Guide, a network driver should install a
185 callback function for the global EXIT_BOOT_SERVICES event (a special NOTIFY
186 type event). When the ExitBootServices() boot service has cleaned up internal
187 firmware state and is about to pass control to the OS, any network driver has
188 to stop any in-flight DMA transfers, lest it corrupts OS memory. For this
189 reason EXIT_BOOT_SERVICES is emitted and the network driver must abort
190 in-flight DMA transfers.
191
192 This callback (VirtioNetExitBoot) is synchronized with the rest of the driver
193 code just the same as explained for WaitForPacket. In
194 EfiSimpleNetworkInitialized state it resets the virtio NIC, halting all data
195 transfer. After the callback returns, no further driver code is expected to
196 be scheduled.
197
198
199 Virtio internals -- Rx
200 ----------------------
201
202 Requests (Rx and Tx alike) are always submitted by the guest and processed by
203 the host. For Tx, processing means transmission. For Rx, processing means
204 filling in the request with an incoming packet. Submitted requests exist on the
205 "Available Ring", and answered (processed) requests show up on the "Used Ring".
206
207 Packet data includes the media (Ethernet) header: destination MAC, source MAC,
208 and Ethertype (14 bytes total).
209
210 The following structures implement packet reception. Most of them are defined
211 in the Virtio specification, the only driver-specific trait here is the static
212 pre-configuration of the two-part descriptor chains, in VirtioNetInitRx. The
213 diagram is simplified.
214
215 Available Index Available Index
216 last processed incremented
217 by the host by the guest
218 v -------> v
219 Available +-------+-------+-------+-------+-------+
220 Ring |DescIdx|DescIdx|DescIdx|DescIdx|DescIdx|
221 +-------+-------+-------+-------+-------+
222 =D6 =D2
223
224 D2 D3 D4 D5 D6 D7
225 Descr. +----------+----------++----------+----------++----------+----------+
226 Table |Adr:Len:Nx|Adr:Len:Nx||Adr:Len:Nx|Adr:Len:Nx||Adr:Len:Nx|Adr:Len:Nx|
227 +----------+----------++----------+----------++----------+----------+
228 =A2 =D3 =A3 =A4 =D5 =A5 =A6 =D7 =A7
229
230
231 A2 A3 A4 A5 A6 A7
232 Receive +---------------+---------------+---------------+
233 Destination |vnet hdr:packet|vnet hdr:packet|vnet hdr:packet|
234 Area +---------------+---------------+---------------+
235
236 Used Index Used Index incremented
237 last processed by the guest by the host
238 v -------> v
239 Used +-----------+-----------+-----------+-----------+-----------+
240 Ring |DescIdx:Len|DescIdx:Len|DescIdx:Len|DescIdx:Len|DescIdx:Len|
241 +-----------+-----------+-----------+-----------+-----------+
242 =D4
243
244 In VirtioNetInitRx, the guest allocates the fixed size Receive Destination
245 Area, which accommodates all packets delivered asynchronously by the host. To
246 each packet, a slice of this area is dedicated; each slice is further
247 subdivided into virtio-net request header and network packet data. The
248 (guest-physical) addresses of these sub-slices are denoted with A2, A3, A4 and
249 so on. Importantly, an even-subscript "A" always belongs to a virtio-net
250 request header, while an odd-subscript "A" always belongs to a packet
251 sub-slice.
252
253 Furthermore, the guest lays out a static pattern in the Descriptor Table. For
254 each packet that can be in-flight or already arrived from the host,
255 VirtioNetInitRx sets up a separate, two-part descriptor chain. For packet N,
256 the Nth descriptor chain is set up as follows:
257
258 - the first (=head) descriptor, with even index, points to the fixed-size
259 sub-slice receiving the virtio-net request header,
260
261 - the second descriptor (with odd index) points to the fixed (1514 byte) size
262 sub-slice receiving the packet data,
263
264 - a link from the first (head) descriptor in the chain is established to the
265 second (tail) descriptor in the chain.
266
267 Finally, the guest populates the Available Ring with the indices of the head
268 descriptors. All descriptor indices on both the Available Ring and the Used
269 Ring are even.
270
271 Packet reception occurs as follows:
272
273 - The host consumes a descriptor index off the Available Ring. This index is
274 even (=2*N), and fingers the head descriptor of the chain belonging to packet
275 N.
276
277 - The host reads the descriptors D(2*N) and -- following the Next link there
278 --- D(2*N+1), and stores the virtio-net request header at A(2*N), and the
279 packet data at A(2*N+1).
280
281 - The host places the index of the head descriptor, 2*N, onto the Used Ring,
282 and sets the Len field in the same Used Ring Element to the total number of
283 bytes transferred for the entire descriptor chain. This enables the guest to
284 identify the length of Rx packets.
285
286 - VirtioNetReceive polls the Used Ring. If a new Used Ring Element shows up, it
287 copies the data out to the caller, and recycles the index of the head
288 descriptor (ie. 2*N) to the Available Ring.
289
290 - Because the host can process (answer) Rx requests in any order theoretically,
291 the order of head descriptor indices on each of the Available Ring and the
292 Used Ring is virtually random. (Except right after the initial population in
293 VirtioNetInitRx, when the Available Ring is full and increasing, and the Used
294 Ring is empty.)
295
296 - If the Available Ring is empty, the host is forced to drop packets. If the
297 Used Ring is empty, VirtioNetReceive returns EFI_NOT_READY (no packet
298 available).
299
300
301 Virtio internals -- Tx
302 ----------------------
303
304 The transmission structure erected by VirtioNetInitTx is similar, it differs
305 in the following:
306
307 - There is no Receive Destination Area.
308
309 - Each head descriptor, D(2*N), points to a read-only virtio-net request header
310 that is shared by all of the head descriptors. This virtio-net request header
311 is never modified by the host.
312
313 - Each tail descriptor is re-pointed to the caller-supplied packet buffer
314 whenever VirtioNetTransmit places the corresponding head descriptor on the
315 Available Ring. The caller is responsible to hang on to the unmodified buffer
316 until it is reported transmitted by VirtioNetGetStatus.
317
318 Steps of packet transmission:
319
320 - Client code calls VirtioNetTransmit. VirtioNetTransmit tracks free descriptor
321 chains by keeping the indices of their head descriptors in a stack that is
322 private to the driver instance. All elements of the stack are even.
323
324 - If the stack is empty (that is, each descriptor chain, in isolation, is
325 either pending transmission, or has been processed by the host but not
326 yet recycled by a VirtioNetGetStatus call), then VirtioNetTransmit returns
327 EFI_NOT_READY.
328
329 - Otherwise the index of a free chain's head descriptor is popped from the
330 stack. The linked tail descriptor is re-pointed as discussed above. The head
331 descriptor's index is pushed on the Available Ring.
332
333 - The host moves the head descriptor index from the Available Ring to the Used
334 Ring when it transmits the packet.
335
336 - Client code calls VirtioNetGetStatus. In case the Used Ring is empty, the
337 function reports no Tx completion. Otherwise, a head descriptor's index is
338 consumed from the Used Ring and recycled to the private stack. The client
339 code's original packet buffer address is fetched from the tail descriptor
340 (where it has been stored at VirtioNetTransmit time) and returned to the
341 caller.
342
343 - The Len field of the Used Ring Element is not checked. The host is assumed to
344 have transmitted the entire packet -- VirtioNetTransmit had forced it below
345 1514 bytes (inclusive). The Virtio specification suggests this packet size is
346 always accepted (and a lower MTU could be encountered on any later hop as
347 well). Additionally, there's no good way to report a short transmit via
348 VirtioNetGetStatus; EFI_DEVICE_ERROR seems too serious from the specification
349 and higher level protocols could interpret it as a fatal condition.
350
351 - The host can theoretically reorder head descriptor indices when moving them
352 from the Available Ring to the Used Ring (out of order transmission). Because
353 of this (and the choice of a stack over a list for free descriptor chain
354 tracking) the order of head descriptor indices on either Ring is
355 unpredictable.