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+AF_XDP device driver
+====================
+
+This driver relies on Linux AF_XDP socket to rx/tx Ethernet packets.
+
+Maturity level
+--------------
+
+Under development: it should work, but has not been thoroughly tested.
+
+Features
+--------
+
+-  copy and zero-copy mode
+-  multiqueue
+-  API
+-  custom eBPF program
+-  polling, interrupt and adaptive mode
+
+Known limitations
+-----------------
+
+MTU
+~~~
+
+Because of AF_XDP restrictions, the MTU is limited to below PAGE_SIZE
+(4096-bytes on most systems) minus 256-bytes, and they are additional
+limitations depending upon specific Linux device drivers. As a rule of
+thumb, a MTU of 3000-bytes or less should be safe.
+
+Number of buffers
+~~~~~~~~~~~~~~~~~
+
+Furthermore, upon UMEM creation, the kernel allocates a
+physically-contiguous structure, whose size is proportional to the
+number of 4KB pages contained in the UMEM. That allocation might fail
+when the number of buffers allocated by VPP is too high. That number can
+be controlled with the ``buffers { buffers-per-numa }`` configuration
+option. Finally, note that because of this limitation, this plugin is
+unlikely to be compatible with the use of 1GB hugepages.
+
+Interrupt mode
+~~~~~~~~~~~~~~
+
+Interrupt and adaptive mode are supported but is limited by default to
+single threaded (no worker) configurations because of a kernel
+limitation prior to 5.6. You can bypass the limitation at interface
+creation time by adding the ``no-syscall-lock`` parameter, but you must
+be sure that your kernel can support it, otherwise you will experience
+double-frees. See
+https://lore.kernel.org/bpf/BYAPR11MB365382C5DB1E5FCC53242609C1549@BYAPR11MB3653.namprd11.prod.outlook.com/
+for more details.
+
+Mellanox
+~~~~~~~~
+
+When setting the number of queues on Mellanox NIC with ``ethtool -L``,
+you must use twice the amount of configured queues: it looks like the
+Linux driver will create separate RX queues and TX queues (but all
+queues can be used for both RX and TX, the NIC will just not sent any
+packet on “pure” TX queues. Confused? So I am.). For example if you set
+``combined 2`` you will effectively have to create 4 rx queues in AF_XDP
+if you want to be sure to receive all packets.
+
+Requirements
+------------
+
+This drivers supports Linux kernel 5.4 and later. Kernels older than 5.4
+are missing unaligned buffers support.
+
+The Linux kernel interface must be up and have enough queues before
+creating the VPP AF_XDP interface, otherwise Linux will deny creating
+the AF_XDP socket. The AF_XDP interface will claim NIC RX queue starting
+from 0, up to the requested number of RX queues (only 1 by default). It
+means all packets destined to NIC RX queue ``[0, num_rx_queues[`` will
+be received by the AF_XDP interface, and only them. Depending on your
+configuration, there will usually be several RX queues (typically 1 per
+core) and packets are spread across queues by RSS. In order to receive
+consistent traffic, you **must** program the NIC dispatching
+accordingly. The simplest way to get all the packets is to specify
+``num-rx-queues all`` to grab all available queues or to reconfigure the
+Linux kernel driver to use only ``num_rx_queues`` RX queues (i.e. all NIC
+queues will be associated with the AF_XDP socket):
+
+::
+
+   ~# ethtool -L <iface> combined <num_rx_queues>
+
+Additionally, the VPP AF_XDP interface will use a MAC address generated
+at creation time instead of the Linux kernel interface MAC. As Linux
+kernel interface are not in promiscuous mode by default (see below) this
+will results in a useless configuration where the VPP AF_XDP interface
+only receives packets destined to the Linux kernel interface MAC just to
+drop them because the destination MAC does not match VPP AF_XDP
+interface MAC. If you want to use the Linux interface MAC for the VPP
+AF_XDP interface, you can change it afterwards in VPP:
+
+::
+
+   ~# vppctl set int mac address <iface> <mac>
+
+Finally, if you wish to receive all packets and not only the packets
+destined to the Linux kernel interface MAC you need to set the Linux
+kernel interface in promiscuous mode:
+
+::
+
+   ~# ip link set dev <iface> promisc on
+
+Security considerations
+-----------------------
+
+When creating an AF_XDP interface, it will receive all packets arriving
+to the NIC RX queue ``[0, num_rx_queues[``. You need to configure the
+Linux kernel NIC driver properly to ensure that only intended packets
+will arrive in this queue. There is no way to filter the packets
+after-the-fact using e.g. netfilter or eBPF.
+
+Quickstart
+----------
+
+1. Put the Linux kernel interface up and in promiscuous mode:
+
+::
+
+   ~# ip l set dev enp216s0f0 promisc on up
+
+2. Create the AF_XDP interface:
+
+::
+
+   ~# vppctl create int af_xdp host-if enp216s0f0 num-rx-queues all
+
+3. Use the interface as usual, e.g.:
+
+::
+
+   ~# vppctl set int ip addr enp216s0f0/0 1.1.1.1/24
+   ~# vppctl set int st enp216s0f0/0 up
+   ~# vppctl ping 1.1.1.100`
+
+Custom eBPF XDP program
+-----------------------
+
+This driver relies on libbpf and as such relies on the ``xsks_map`` eBPF
+map. The default behavior is to use the XDP program already attached to
+the interface if any, otherwise load the default one. You can request to
+load a custom XDP program with the ``prog`` option when creating the
+interface in VPP:
+
+::
+
+   ~# vppctl create int af_xdp host-if enp216s0f0 num-rx-queues 4 prog extras/bpf/af_xdp.bpf.o
+
+In that case it will replace any previously attached program. A custom
+XDP program example is provided in ``extras/bpf/``.
+
+Performance consideration
+-------------------------
+
+AF_XDP relies on the Linux kernel NIC driver to rx/tx packets. To reach
+high-performance (10’s MPPS), the Linux kernel NIC driver must support
+zero-copy mode and its RX path must run on a dedicated core in the NUMA
+where the NIC is physically connected.