XDP is an eBPF based high performance data path merged in the Linux kernel since version 4.8. The idea behind XDP is to add an early hook in the RX path of the kernel, and let a user supplied eBPF program decide the fate of the packet. The hook is placed in the NICdriver just after the interrupt processing, and before any memory allocation needed by the network stack itself, because memory allocation can be an expensive operation. Due to this design, XDP can drop 26 millions of packets per secondper core with commodity hardware. The eBPF program must pass a preverifier test before being loaded, to avoid executing malicious code in kernel space. The preverifier checks that the program contains no out-of-bounds accesses, loops or global variables. The program is allowed to edit the packet data and, after the eBPF program returns, an action code determines what to do with the packet:
XDP_PASS: let the packet continue through the network stack
XDP_DROP: silently drop the packet
XDP_ABORTED: drop the packet with trace point exception
XDP_TX: bounce the packet back to the same NIC it arrived on
XDP_REDIRECT: redirect the packet to another NIC or user spacesocket via the AF_XDP address family
XDP requires support in the NIC driver but, as not all drivers supports it, it can fallback to a generic implementation, which performs the eBPF processing in the network stack, though with slower performances. XDP has infrastructure to offload the eBPF program to a NIC card which supports it, reducing the CPU load. At the time only Netronomecards supports it, with Intel and Mellanox working on it.
AF_XDP
Along with XDP, a new address family entered in the Linux kernel starting 4.18. AF_XDP, formerly known as AF_PACKETv4, is a raw socket optimized for high performance packet processing and allows zero-copy between kernel and applications. As the socket can be used for both receiving and transmitting, it supports high performance network applications purely in user space.
