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diff --git a/Documentation/networking/representors.rst b/Documentation/networking/representors.rst new file mode 100644 index 0000000000..decb39c19b --- /dev/null +++ b/Documentation/networking/representors.rst @@ -0,0 +1,261 @@ +.. 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, the driver should use the ``SET_NETDEV_DEVLINK_PORT`` macro to +assign a devlink port instance to the netdevice before registering the +netdevice; the kernel uses the devlink port 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>`) |