OvmfPkg/VirtioNetDxe/TechNotes.txt

   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.