## @file
#
# Technical notes for the virtio-net driver.
#
# Copyright (C) 2013, Red Hat, Inc.
#
# SPDX-License-Identifier: BSD-2-Clause-Patent
#
##

Disclaimer
----------

All statements concerning standards and specifications are informative and not
normative. They are made in good faith. Corrections are most welcome on the
edk2-devel mailing list.

The following documents have been perused while writing the driver and this
document:
- Unified Extensible Firmware Interface Specification, Version 2.3.1, Errata C;
  June 27, 2012
- Driver Writer's Guide for UEFI 2.3.1, 03/08/2012, Version 1.01;
- Virtio PCI Card Specification, v0.9.5 DRAFT, 2012 May 7.


Summary
-------

The VirtioNetDxe UEFI_DRIVER implements the Simple Network Protocol for
virtio-net devices. Higher level protocols are automatically installed on top
of it by the DXE Core / the ConnectController() boot service, enabling for
virtio-net devices eg. DHCP configuration, TCP transfers with edk2 StdLib
applications, and PXE booting in OVMF.


UEFI driver structure
---------------------

A driver instance, belonging to a given virtio-net device, can be in one of
four states at any time. The states stack up as follows below. The state
transitions are labeled with the primary function (and its important callees
faithfully indented) that implement the transition.

                               |  ^
                               |  |
   [DriverBinding.c]           |  | [DriverBinding.c]
   VirtioNetDriverBindingStart |  | VirtioNetDriverBindingStop
     VirtioNetSnpPopulate      |  |   VirtioNetSnpEvacuate
       VirtioNetGetFeatures    |  |
                               v  |
                   +-------------------------+
                   | EfiSimpleNetworkStopped |
                   +-------------------------+
                               |  ^
                [SnpStart.c]   |  | [SnpStop.c]
                VirtioNetStart |  | VirtioNetStop
                               |  |
                               v  |
                   +-------------------------+
                   | EfiSimpleNetworkStarted |
                   +-------------------------+
                               |  ^
  [SnpInitialize.c]            |  | [SnpShutdown.c]
  VirtioNetInitialize          |  | VirtioNetShutdown
    VirtioNetInitRing {Rx, Tx} |  |   VirtioNetShutdownRx [SnpSharedHelpers.c]
      VirtioRingInit           |  |     VirtIo->UnmapSharedBuffer
      VirtioRingMap            |  |     VirtIo->FreeSharedPages
    VirtioNetInitTx            |  |   VirtioNetShutdownTx [SnpSharedHelpers.c]
      VirtIo->AllocateShare... |  |     VirtIo->UnmapSharedBuffer
      VirtioMapAllBytesInSh... |  |     VirtIo->FreeSharedPages
    VirtioNetInitRx            |  |   VirtioNetUninitRing [SnpSharedHelpers.c]
      VirtIo->AllocateShare... |  |                       {Tx, Rx}
      VirtioMapAllBytesInSh... |  |     VirtIo->UnmapSharedBuffer
                               |  |     VirtioRingUninit
                               v  |
                  +-----------------------------+
                  | EfiSimpleNetworkInitialized |
                  +-----------------------------+

The state at the top means "nonexistent" and is hence unnamed on the diagram --
a driver instance actually doesn't exist at that point. The transition
functions out of and into that state implement the Driver Binding Protocol.

The lower three states characterize an existent driver instance and are all
states defined by the Simple Network Protocol. The transition functions between
them are member functions of the Simple Network Protocol.

Each transition function validates its expected source state and its
parameters. For example, VirtioNetDriverBindingStop will refuse to disconnect
from the controller unless it's in EfiSimpleNetworkStopped.


Driver instance states (Simple Network Protocol)
------------------------------------------------

In the EfiSimpleNetworkStopped state, the virtio-net device is (has been)
re-set. No resources are allocated for networking / traffic purposes. The MAC
address and other device attributes have been retrieved from the device (this
is necessary for completing the VirtioNetDriverBindingStart transition).

The EfiSimpleNetworkStarted is completely identical to the
EfiSimpleNetworkStopped state for virtio-net, in the functional and
resource-usage sense. This state is mandated / provided by the Simple Network
Protocol for flexibility that the virtio-net driver doesn't exploit.

In particular, the EfiSimpleNetworkStarted state is the target of the Shutdown
SNP member function, and must therefore correspond to a hardware configuration
where "[it] is safe for another driver to initialize". (Clearly another UEFI
driver could not do that due to the exclusivity of the driver binding that
VirtioNetDriverBindingStart() installs, but a later OS driver might qualify.)

The EfiSimpleNetworkInitialized state is the live state of the virtio NIC / the
driver instance. Virtio and other resources required for network traffic have
been allocated, and the following SNP member functions are available (in
addition to VirtioNetShutdown which leaves the state):

- VirtioNetReceive [SnpReceive.c]: poll the virtio NIC for an Rx packet that
  may have arrived asynchronously;

- VirtioNetTransmit [SnpTransmit.c]: queue a Tx packet for asynchronous
  transmission (meant to be used together with VirtioNetGetStatus);

- VirtioNetGetStatus [SnpGetStatus.c]: query link status and status of pending
  Tx packets;

- VirtioNetMcastIpToMac [SnpMcastIpToMac.c]: transform a multicast IPv4/IPv6
  address into a multicast MAC address;

- VirtioNetReceiveFilters [SnpReceiveFilters.c]: emulate unicast / multicast /
  broadcast filter configuration (not their actual effect -- a more liberal
  filter setting than requested is allowed by the UEFI specification).

The following SNP member functions are not supported [SnpUnsupported.c]:

- VirtioNetReset: reinitialize the virtio NIC without shutting it down (a loop
  from/to EfiSimpleNetworkInitialized);

- VirtioNetStationAddress: assign a new MAC address to the virtio NIC,

- VirtioNetStatistics: collect statistics,

- VirtioNetNvData: access non-volatile data on the virtio NIC.

Missing support for these functions is allowed by the UEFI specification and
doesn't seem to trip up higher level protocols.


Events and task priority levels
-------------------------------

The UEFI specification defines a sophisticated mechanism for asynchronous
events / callbacks (see "6.1 Event, Timer, and Task Priority Services" for
details). Such callbacks work like software interrupts, and some notion of
locking / masking is important to implement critical sections (atomic or
exclusive access to data or a device). This notion is defined as Task Priority
Levels.

The virtio-net driver for OVMF must concern itself with events for two reasons:

- The Simple Network Protocol provides its clients with a (non-optional) WAIT
  type event called WaitForPacket: it allows them to check or wait for Rx
  packets by polling or blocking on this event. (This functionality overlaps
  with the Receive member function.) The event is available to clients starting
  with EfiSimpleNetworkStopped (inclusive).

  The virtio-net driver is informed about such client polling or blockage by
  receiving an asynchronous callback (a software interrupt). In the callback
  function the driver must interrogate the driver instance state, and if it is
  EfiSimpleNetworkInitialized, access the Rx queue and see if any packets are
  available for consumption. If so, it must signal the WaitForPacket WAIT type
  event, waking the client.

  For simplicity and safety, all parts of the virtio-net driver that access any
  bit of the driver instance (data or device) run at the TPL_CALLBACK level.
  This is the highest level allowed for an SNP implementation, and all code
  protected in this manner satisfies even stricter non-blocking requirements
  than what's documented for TPL_CALLBACK.

  The task priority level for the WaitForPacket callback too is set by the
  driver, the choice is TPL_CALLBACK again. This in effect serializes  the
  WaitForPacket callback (VirtioNetIsPacketAvailable [Events.c]) with "normal"
  parts of the driver.

- According to the Driver Writer's Guide, a network driver should install a
  callback function for the global EXIT_BOOT_SERVICES event (a special NOTIFY
  type event). When the ExitBootServices() boot service has cleaned up internal
  firmware state and is about to pass control to the OS, any network driver has
  to stop any in-flight DMA transfers, lest it corrupts OS memory. For this
  reason EXIT_BOOT_SERVICES is emitted and the network driver must abort
  in-flight DMA transfers.

  This callback (VirtioNetExitBoot) is synchronized with the rest of the driver
  code just the same as explained for WaitForPacket. In
  EfiSimpleNetworkInitialized state it resets the virtio NIC, halting all data
  transfer. After the callback returns, no further driver code is expected to
  be scheduled.


Virtio internals -- Rx
----------------------

Requests (Rx and Tx alike) are always submitted by the guest and processed by
the host. For Tx, processing means transmission. For Rx, processing means
filling in the request with an incoming packet. Submitted requests exist on the
"Available Ring", and answered (processed) requests show up on the "Used Ring".

Packet data includes the media (Ethernet) header: destination MAC, source MAC,
and Ethertype (14 bytes total).

The following structures implement packet reception. Most of them are defined
in the Virtio specification, the only driver-specific trait here is the static
pre-configuration of the two-part descriptor chains, in VirtioNetInitRx. The
diagram is simplified.

                     Available Index       Available Index
                     last processed          incremented
                       by the host           by the guest
                           v       ------->        v
Available  +-------+-------+-------+-------+-------+
Ring       |DescIdx|DescIdx|DescIdx|DescIdx|DescIdx|
           +-------+-------+-------+-------+-------+
                              =D6     =D2

       D2         D3          D4         D5          D6         D7
Descr. +----------+----------++----------+----------++----------+----------+
Table  |Adr:Len:Nx|Adr:Len:Nx||Adr:Len:Nx|Adr:Len:Nx||Adr:Len:Nx|Adr:Len:Nx|
       +----------+----------++----------+----------++----------+----------+
        =A2    =D3 =A3         =A4    =D5 =A5         =A6    =D7 =A7


            A2        A3     A4       A5     A6       A7
Receive     +---------------+---------------+---------------+
Destination |vnet hdr:packet|vnet hdr:packet|vnet hdr:packet|
Area        +---------------+---------------+---------------+

                Used Index                               Used Index incremented
        last processed by the guest                            by the host
                    v                    ------->                   v
Used    +-----------+-----------+-----------+-----------+-----------+
Ring    |DescIdx:Len|DescIdx:Len|DescIdx:Len|DescIdx:Len|DescIdx:Len|
        +-----------+-----------+-----------+-----------+-----------+
                                     =D4

In VirtioNetInitRx, the guest allocates the fixed size Receive Destination
Area, which accommodates all packets delivered asynchronously by the host. To
each packet, a slice of this area is dedicated; each slice is further
subdivided into virtio-net request header and network packet data. The
(device-physical) addresses of these sub-slices are denoted with A2, A3, A4 and
so on. Importantly, an even-subscript "A" always belongs to a virtio-net
request header, while an odd-subscript "A" always belongs to a packet
sub-slice.

Furthermore, the guest lays out a static pattern in the Descriptor Table. For
each packet that can be in-flight or already arrived from the host,
VirtioNetInitRx sets up a separate, two-part descriptor chain. For packet N,
the Nth descriptor chain is set up as follows:

- the first (=head) descriptor, with even index, points to the fixed-size
  sub-slice receiving the virtio-net request header,

- the second descriptor (with odd index) points to the fixed (1514 byte) size
  sub-slice receiving the packet data,

- a link from the first (head) descriptor in the chain is established to the
  second (tail) descriptor in the chain.

Finally, the guest populates the Available Ring with the indices of the head
descriptors. All descriptor indices on both the Available Ring and the Used
Ring are even.

Packet reception occurs as follows:

- The host consumes a descriptor index off the Available Ring. This index is
  even (=2*N), and fingers the head descriptor of the chain belonging to packet
  N.

- The host reads the descriptors D(2*N) and -- following the Next link there
  --- D(2*N+1), and stores the virtio-net request header at A(2*N), and the
  packet data at A(2*N+1).

- The host places the index of the head descriptor, 2*N, onto the Used Ring,
  and sets the Len field in the same Used Ring Element to the total number of
  bytes transferred for the entire descriptor chain. This enables the guest to
  identify the length of Rx packets.

- VirtioNetReceive polls the Used Ring. If a new Used Ring Element shows up, it
  copies the data out to the caller, and recycles the index of the head
  descriptor (ie. 2*N) to the Available Ring.

- Because the host can process (answer) Rx requests in any order theoretically,
  the order of head descriptor indices on each of the Available Ring and the
  Used Ring is virtually random. (Except right after the initial population in
  VirtioNetInitRx, when the Available Ring is full and increasing, and the Used
  Ring is empty.)

- If the Available Ring is empty, the host is forced to drop packets. If the
  Used Ring is empty, VirtioNetReceive returns EFI_NOT_READY (no packet
  available).


Virtio internals -- Tx
----------------------

The transmission structure erected by VirtioNetInitTx is similar, it differs
in the following:

- There is no Receive Destination Area.

- Each head descriptor, D(2*N), points to a read-only virtio-net request header
  that is shared by all of the head descriptors. This virtio-net request header
  is never modified by the host.

- Each tail descriptor is re-pointed to the device-mapped address of the
  caller-supplied packet buffer whenever VirtioNetTransmit places the
  corresponding head descriptor on the Available Ring. A reverse mapping, from
  the device-mapped address to the caller-supplied packet address, is saved in
  an associative data structure that belongs to the driver instance.

- Per spec, the caller is responsible to hang on to the unmodified packet
  buffer until it is reported transmitted by VirtioNetGetStatus.

Steps of packet transmission:

- Client code calls VirtioNetTransmit. VirtioNetTransmit tracks free descriptor
  chains by keeping the indices of their head descriptors in a stack that is
  private to the driver instance. All elements of the stack are even.

- If the stack is empty (that is, each descriptor chain, in isolation, is
  either pending transmission, or has been processed by the host but not
  yet recycled by a VirtioNetGetStatus call), then VirtioNetTransmit returns
  EFI_NOT_READY.

- Otherwise the index of a free chain's head descriptor is popped from the
  stack. The linked tail descriptor is re-pointed as discussed above. The head
  descriptor's index is pushed on the Available Ring.

- The host moves the head descriptor index from the Available Ring to the Used
  Ring when it transmits the packet.

- Client code calls VirtioNetGetStatus. In case the Used Ring is empty, the
  function reports no Tx completion. Otherwise, a head descriptor's index is
  consumed from the Used Ring and recycled to the private stack. The client
  code's original packet buffer address is calculated by fetching the
  device-mapped address from the tail descriptor (where it has been stored at
  VirtioNetTransmit time), and by looking up the device-mapped address in the
  associative data structure. The reverse-mapped packet buffer address is
  returned to the caller.

- The Len field of the Used Ring Element is not checked. The host is assumed to
  have transmitted the entire packet -- VirtioNetTransmit had forced it below
  1514 bytes (inclusive). The Virtio specification suggests this packet size is
  always accepted (and a lower MTU could be encountered on any later hop as
  well). Additionally, there's no good way to report a short transmit via
  VirtioNetGetStatus; EFI_DEVICE_ERROR seems too serious from the specification
  and higher level protocols could interpret it as a fatal condition.

- The host can theoretically reorder head descriptor indices when moving them
  from the Available Ring to the Used Ring (out of order transmission). Because
  of this (and the choice of a stack over a list for free descriptor chain
  tracking) the order of head descriptor indices on either Ring is
  unpredictable.