docs: net: add an explanation of VF (and other) Representors

There's no clear explanation of what VF Representors are for, their
 semantics, etc., outside of vendor docs and random conference slides.
Add a document explaining Representors and defining what drivers that
 implement them are expected to do.
Signed-off-by: NEdward Cree <ecree.xilinx@gmail.com>
Reviewed-by: NBagas Sanjaya <bagasdotme@gmail.com>
Link: https://lore.kernel.org/r/20220905135557.39233-1-ecree@xilinx.comSigned-off-by: NJakub Kicinski <kuba@kernel.org>

Documentation/networking/index.rst

@@ -92,6 +92,7 @@ Contents:
    radiotap-headers
    rds
    regulatory
+   representors
    rxrpc
    sctp
    secid

Documentation/networking/representors.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+=============================
+Network Function Representors
+=============================
+
+This document describes the semantics and usage of representor netdevices, as
+used to control internal switching on SmartNICs.  For the closely-related port
+representors on physical (multi-port) switches, see
+:ref:`Documentation/networking/switchdev.rst <switchdev>`.
+
+Motivation
+----------
+
+Since the mid-2010s, network cards have started offering more complex
+virtualisation capabilities than the legacy SR-IOV approach (with its simple
+MAC/VLAN-based switching model) can support.  This led to a desire to offload
+software-defined networks (such as OpenVSwitch) to these NICs to specify the
+network connectivity of each function.  The resulting designs are variously
+called SmartNICs or DPUs.
+
+Network function representors bring the standard Linux networking stack to
+virtual switches and IOV devices.  Just as each physical port of a Linux-
+controlled switch has a separate netdev, so does each virtual port of a virtual
+switch.
+When the system boots, and before any offload is configured, all packets from
+the virtual functions appear in the networking stack of the PF via the
+representors.  The PF can thus always communicate freely with the virtual
+functions.
+The PF can configure standard Linux forwarding between representors, the uplink
+or any other netdev (routing, bridging, TC classifiers).
+
+Thus, a representor is both a control plane object (representing the function in
+administrative commands) and a data plane object (one end of a virtual pipe).
+As a virtual link endpoint, the representor can be configured like any other
+netdevice; in some cases (e.g. link state) the representee will follow the
+representor's configuration, while in others there are separate APIs to
+configure the representee.
+
+Definitions
+-----------
+
+This document uses the term "switchdev function" to refer to the PCIe function
+which has administrative control over the virtual switch on the device.
+Typically, this will be a PF, but conceivably a NIC could be configured to grant
+these administrative privileges instead to a VF or SF (subfunction).
+Depending on NIC design, a multi-port NIC might have a single switchdev function
+for the whole device or might have a separate virtual switch, and hence
+switchdev function, for each physical network port.
+If the NIC supports nested switching, there might be separate switchdev
+functions for each nested switch, in which case each switchdev function should
+only create representors for the ports on the (sub-)switch it directly
+administers.
+
+A "representee" is the object that a representor represents.  So for example in
+the case of a VF representor, the representee is the corresponding VF.
+
+What does a representor do?
+---------------------------
+
+A representor has three main roles.
+
+1. It is used to configure the network connection the representee sees, e.g.
+   link up/down, MTU, etc.  For instance, bringing the representor
+   administratively UP should cause the representee to see a link up / carrier
+   on event.
+2. It provides the slow path for traffic which does not hit any offloaded
+   fast-path rules in the virtual switch.  Packets transmitted on the
+   representor netdevice should be delivered to the representee; packets
+   transmitted by the representee which fail to match any switching rule should
+   be received on the representor netdevice.  (That is, there is a virtual pipe
+   connecting the representor to the representee, similar in concept to a veth
+   pair.)
+   This allows software switch implementations (such as OpenVSwitch or a Linux
+   bridge) to forward packets between representees and the rest of the network.
+3. It acts as a handle by which switching rules (such as TC filters) can refer
+   to the representee, allowing these rules to be offloaded.
+
+The combination of 2) and 3) means that the behaviour (apart from performance)
+should be the same whether a TC filter is offloaded or not.  E.g. a TC rule
+on a VF representor applies in software to packets received on that representor
+netdevice, while in hardware offload it would apply to packets transmitted by
+the representee VF.  Conversely, a mirred egress redirect to a VF representor
+corresponds in hardware to delivery directly to the representee VF.
+
+What functions should have a representor?
+-----------------------------------------
+
+Essentially, for each virtual port on the device's internal switch, there
+should be a representor.
+Some vendors have chosen to omit representors for the uplink and the physical
+network port, which can simplify usage (the uplink netdev becomes in effect the
+physical port's representor) but does not generalise to devices with multiple
+ports or uplinks.
+
+Thus, the following should all have representors:
+
+ - VFs belonging to the switchdev function.
+ - Other PFs on the local PCIe controller, and any VFs belonging to them.
+ - PFs and VFs on external PCIe controllers on the device (e.g. for any embedded
+   System-on-Chip within the SmartNIC).
+ - PFs and VFs with other personalities, including network block devices (such
+   as a vDPA virtio-blk PF backed by remote/distributed storage), if (and only
+   if) their network access is implemented through a virtual switch port. [#]_
+   Note that such functions can require a representor despite the representee
+   not having a netdev.
+ - Subfunctions (SFs) belonging to any of the above PFs or VFs, if they have
+   their own port on the switch (as opposed to using their parent PF's port).
+ - Any accelerators or plugins on the device whose interface to the network is
+   through a virtual switch port, even if they do not have a corresponding PCIe
+   PF or VF.
+
+This allows the entire switching behaviour of the NIC to be controlled through
+representor TC rules.
+
+It is a common misunderstanding to conflate virtual ports with PCIe virtual
+functions or their netdevs.  While in simple cases there will be a 1:1
+correspondence between VF netdevices and VF representors, more advanced device
+configurations may not follow this.
+A PCIe function which does not have network access through the internal switch
+(not even indirectly through the hardware implementation of whatever services
+the function provides) should *not* have a representor (even if it has a
+netdev).
+Such a function has no switch virtual port for the representor to configure or
+to be the other end of the virtual pipe.
+The representor represents the virtual port, not the PCIe function nor the 'end
+user' netdevice.
+
+.. [#] The concept here is that a hardware IP stack in the device performs the
+   translation between block DMA requests and network packets, so that only
+   network packets pass through the virtual port onto the switch.  The network
+   access that the IP stack "sees" would then be configurable through tc rules;
+   e.g. its traffic might all be wrapped in a specific VLAN or VxLAN.  However,
+   any needed configuration of the block device *qua* block device, not being a
+   networking entity, would not be appropriate for the representor and would
+   thus use some other channel such as devlink.
+   Contrast this with the case of a virtio-blk implementation which forwards the
+   DMA requests unchanged to another PF whose driver then initiates and
+   terminates IP traffic in software; in that case the DMA traffic would *not*
+   run over the virtual switch and the virtio-blk PF should thus *not* have a
+   representor.
+
+How are representors created?
+-----------------------------
+
+The driver instance attached to the switchdev function should, for each virtual
+port on the switch, create a pure-software netdevice which has some form of
+in-kernel reference to the switchdev function's own netdevice or driver private
+data (``netdev_priv()``).
+This may be by enumerating ports at probe time, reacting dynamically to the
+creation and destruction of ports at run time, or a combination of the two.
+
+The operations of the representor netdevice will generally involve acting
+through the switchdev function.  For example, ``ndo_start_xmit()`` might send
+the packet through a hardware TX queue attached to the switchdev function, with
+either packet metadata or queue configuration marking it for delivery to the
+representee.
+
+How are representors identified?
+--------------------------------
+
+The representor netdevice should *not* directly refer to a PCIe device (e.g.
+through ``net_dev->dev.parent`` / ``SET_NETDEV_DEV()``), either of the
+representee or of the switchdev function.
+Instead, it should implement the ``ndo_get_devlink_port()`` netdevice op, which
+the kernel uses to provide the ``phys_switch_id`` and ``phys_port_name`` sysfs
+nodes.  (Some legacy drivers implement ``ndo_get_port_parent_id()`` and
+``ndo_get_phys_port_name()`` directly, but this is deprecated.)  See
+:ref:`Documentation/networking/devlink/devlink-port.rst <devlink_port>` for the
+details of this API.
+
+It is expected that userland will use this information (e.g. through udev rules)
+to construct an appropriately informative name or alias for the netdevice.  For
+instance if the switchdev function is ``eth4`` then a representor with a
+``phys_port_name`` of ``p0pf1vf2`` might be renamed ``eth4pf1vf2rep``.
+
+There are as yet no established conventions for naming representors which do not
+correspond to PCIe functions (e.g. accelerators and plugins).
+
+How do representors interact with TC rules?
+-------------------------------------------
+
+Any TC rule on a representor applies (in software TC) to packets received by
+that representor netdevice.  Thus, if the delivery part of the rule corresponds
+to another port on the virtual switch, the driver may choose to offload it to
+hardware, applying it to packets transmitted by the representee.
+
+Similarly, since a TC mirred egress action targeting the representor would (in
+software) send the packet through the representor (and thus indirectly deliver
+it to the representee), hardware offload should interpret this as delivery to
+the representee.
+
+As a simple example, if ``PORT_DEV`` is the physical port representor and
+``REP_DEV`` is a VF representor, the following rules::
+
+    tc filter add dev $REP_DEV parent ffff: protocol ipv4 flower \
+        action mirred egress redirect dev $PORT_DEV
+    tc filter add dev $PORT_DEV parent ffff: protocol ipv4 flower skip_sw \
+        action mirred egress mirror dev $REP_DEV
+
+would mean that all IPv4 packets from the VF are sent out the physical port, and
+all IPv4 packets received on the physical port are delivered to the VF in
+addition to ``PORT_DEV``.  (Note that without ``skip_sw`` on the second rule,
+the VF would get two copies, as the packet reception on ``PORT_DEV`` would
+trigger the TC rule again and mirror the packet to ``REP_DEV``.)
+
+On devices without separate port and uplink representors, ``PORT_DEV`` would
+instead be the switchdev function's own uplink netdevice.
+
+Of course the rules can (if supported by the NIC) include packet-modifying
+actions (e.g. VLAN push/pop), which should be performed by the virtual switch.
+
+Tunnel encapsulation and decapsulation are rather more complicated, as they
+involve a third netdevice (a tunnel netdev operating in metadata mode, such as
+a VxLAN device created with ``ip link add vxlan0 type vxlan external``) and
+require an IP address to be bound to the underlay device (e.g. switchdev
+function uplink netdev or port representor).  TC rules such as::
+
+    tc filter add dev $REP_DEV parent ffff: flower \
+        action tunnel_key set id $VNI src_ip $LOCAL_IP dst_ip $REMOTE_IP \
+                              dst_port 4789 \
+        action mirred egress redirect dev vxlan0
+    tc filter add dev vxlan0 parent ffff: flower enc_src_ip $REMOTE_IP \
+        enc_dst_ip $LOCAL_IP enc_key_id $VNI enc_dst_port 4789 \
+        action tunnel_key unset action mirred egress redirect dev $REP_DEV
+
+where ``LOCAL_IP`` is an IP address bound to ``PORT_DEV``, and ``REMOTE_IP`` is
+another IP address on the same subnet, mean that packets sent by the VF should
+be VxLAN encapsulated and sent out the physical port (the driver has to deduce
+this by a route lookup of ``LOCAL_IP`` leading to ``PORT_DEV``, and also
+perform an ARP/neighbour table lookup to find the MAC addresses to use in the
+outer Ethernet frame), while UDP packets received on the physical port with UDP
+port 4789 should be parsed as VxLAN and, if their VSID matches ``$VNI``,
+decapsulated and forwarded to the VF.
+
+If this all seems complicated, just remember the 'golden rule' of TC offload:
+the hardware should ensure the same final results as if the packets were
+processed through the slow path, traversed software TC (except ignoring any
+``skip_hw`` rules and applying any ``skip_sw`` rules) and were transmitted or
+received through the representor netdevices.
+
+Configuring the representee's MAC
+---------------------------------
+
+The representee's link state is controlled through the representor.  Setting the
+representor administratively UP or DOWN should cause carrier ON or OFF at the
+representee.
+
+Setting an MTU on the representor should cause that same MTU to be reported to
+the representee.
+(On hardware that allows configuring separate and distinct MTU and MRU values,
+the representor MTU should correspond to the representee's MRU and vice-versa.)
+
+Currently there is no way to use the representor to set the station permanent
+MAC address of the representee; other methods available to do this include:
+
+ - legacy SR-IOV (``ip link set DEVICE vf NUM mac LLADDR``)
+ - devlink port function (see **devlink-port(8)** and
+   :ref:`Documentation/networking/devlink/devlink-port.rst <devlink_port>`)

Documentation/networking/switchdev.rst

 .. SPDX-License-Identifier: GPL-2.0
 .. include:: <isonum.txt>
+.. _switchdev:
 
 ===============================================
 Ethernet switch device driver model (switchdev)