af-xdp

AF_XDP

Purpose

Guide agents through AF_XDP sockets for high-performance packet I/O: socket creation, UMEM setup, fill/completion/RX/TX rings, XDP programs with XDP_REDIRECT to XSK, copy vs zero-copy modes, libbpf helpers, performance comparison with DPDK, and production use cases.

When to Use

• Building a userspace packet processor with lower overhead than raw sockets
• Redirecting XDP-filtered traffic to userspace without DPDK complexity
• Implementing a custom load balancer or IDS dataplane
• Comparing zero-copy vs copy mode on your NIC/driver
• Integrating with existing libbpf/XDP infrastructure
• Need kernel cooperation (firewall rules) plus userspace processing

Workflow

1. Architecture overview

NIC → XDP program (BPF) → XDP_REDIRECT → AF_XDP socket → userspace
                ↓
           XDP_DROP/PASS/TX

Components:

• UMEM — shared memory region for frames
• Fill ring — userspace provides empty frame addresses to kernel
• Completion ring — kernel returns completed TX frames
• RX ring — kernel delivers received packets
• TX ring — userspace submits packets for transmission

2. UMEM and XSK socket creation

#include <bpf/xsk.h>
#include <bpf/libbpf.h>
#include <xdp/xsk.h>

#define NUM_FRAMES     4096
#define FRAME_SIZE     XSK_UMEM__DEFAULT_FRAME_SIZE
#define RX_BATCH_SIZE  64

struct xsk_umem_info {
    struct xsk_ring_prod fill;
    struct xsk_ring_cons comp;
    struct xsk_umem *umem;
    void *buffer;
};

struct xsk_socket_info {
    struct xsk_ring_cons rx;
    struct xsk_ring_prod tx;
    struct xsk_socket *xsk;
};

int xsk_setup(struct xsk_umem_info *umem_info,
              struct xsk_socket_info *xsk_info,
              int ifindex, int queue_id, int xsk_flags)
{
    umem_info->buffer = mmap(NULL, NUM_FRAMES * FRAME_SIZE,
        PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);

    struct xsk_umem_config umem_cfg = {
        .fill_size = XSK_RING_PROD__DEFAULT_NUM_DESCS,
        .comp_size = XSK_RING_CONS__DEFAULT_NUM_DESCS,
        .frame_size = FRAME_SIZE,
        .frame_headroom = XSK_UMEM__DEFAULT_FRAME_HEADROOM,
        .flags = 0,
    };

    int ret = xsk_umem__create(&umem_info->umem, umem_info->buffer,
        NUM_FRAMES * FRAME_SIZE, &umem_info->fill, &umem_info->comp,
        &umem_cfg);
    if (ret)
        return ret;

    struct xsk_socket_config xsk_cfg = {
        .rx_size = XSK_RING_CONS__DEFAULT_NUM_DESCS,
        .tx_size = XSK_RING_PROD__DEFAULT_NUM_DESCS,
        .libbpf_flags = XSK_LIBBPF_FLAGS__INHIBIT_PROG_LOAD,
        .xdp_flags = XDP_FLAGS_UPDATE_IF_NOEXIST,
        .bind_flags = xsk_flags,  // XDP_ZEROCOPY or XDP_COPY
    };

    return xsk_socket__create(&xsk_info->xsk, "eth0", queue_id,
        umem_info->umem, &xsk_info->rx, &xsk_info->tx, &xsk_cfg);
}