af-xdp

AF_XDP sockets

Contract

Inputs

1. Interface and queue (required): the interface name and the RX queue the socket serves. One socket per queue.
2. Mode intent (optional): copy or zero-copy. Default to copy and confirm from the bind.
3. Build setup (optional): libbpf installed (pkg-config --libs libbpf returns -lbpf; -lxdp when libxdp is installed separately).

Procedure

1. Lay out the object model before writing code. UMEM is the shared frame pool. The fill ring carries empty frame addresses from user space to the kernel. The completion ring returns transmitted frames. The RX ring delivers received packets. The TX ring carries outgoing packets. Done when: each ring and the UMEM has a named struct in the design.

2. Create the UMEM and the socket.

   #include <bpf/xsk.h>
   
   #define NUM_FRAMES    4096
   #define FRAME_SIZE    XSK_UMEM__DEFAULT_FRAME_SIZE
   #define RX_BATCH_SIZE 64
   
   /* mmap one anonymous region; the UMEM describes it to the kernel */
   void *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->umem, buffer, NUM_FRAMES * FRAME_SIZE,
                              &umem->fill, &umem->comp, &umem_cfg);
   
   struct xsk_socket_config xsk_cfg = {
       .rx_size = XSK_RING_CONS__DEFAULT_NUM_DESCS,
       .tx_size = XSK_RING_PROD__DEFAULT_NUM_DESCS,
       /* the skill loads its own program in step 4, so stop libbpf */
       .libbpf_flags = XSK_LIBBPF_FLAGS__INHIBIT_PROG_LOAD,
       .xdp_flags = XDP_FLAGS_UPDATE_IF_NOEXIST,
       .bind_flags = XDP_COPY, /* or XDP_ZEROCOPY, see step 6 */
   };
   ret = xsk_socket__create(&xsk->xsk, ifname, queue_id, umem->umem,
                            &xsk->rx, &xsk->tx, &xsk_cfg);