apps.xdp: driver for AF_XDP sockets

src/apps/xdp/README.md

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+# XDP socket app (apps.xdp.xdp)
+
+The `XDP` app implements a driver for Linux `AF_XDP` sockets.
+
+Its links are named `input` and `output`.
+
+    DIAGRAM: XDP
+                 +-----------+
+                 |           |
+      input ---->*    XDP    *----> output
+                 |           |
+                 +-----------+
+
+**Important:** To use the _XDP_ app, “Snabb XDP mode“ must be enabled by
+calling `xdp.snabb_enable_xdp()`. Calling this function replaces Snabb's native
+memory allocator with the _UMEM_ allocator. The caller must ensure that no
+packets have been allocated via `packet.allocate()` prior to calling this
+function.
+
+## _Caveats_
+
+   * Memory allocated by the UMEM allocator can not be used with _DMA_
+     drivers: using the XDP app precludes the use of Snabb’s native
+     hardware drivers such as `apps.intel_mp.intel_mp`.
+
+   * Memory allocated by the UMEM allocator can not be shared with
+     other Snabb processes in the same process group: using
+     snabb_enable_xdp precludes the use of Interlink apps
+     (`apps.interlink`).
+
+## Maximum MTU
+
+Due to a combination of how Snabb uses packet buffers and a limitation of
+`AF_XDP` the effective maximum MTU of the XDP app is 3,582.
+
+## Configuration
+
+— Key **ifname**
+
+*Required*. The name of the interface as shown in `ip link`.
+
+— Key **queue**
+
+*Optional*. Queue to bind to (zero based). The default is queue 0.
+
+## Module functions
+
+— Function **snabb_enable_xdp**
+
+Enables “Snabb XDP mode”. See _Caveats_!
+
+## Setting up XDP capable devices under Linux
+
+```
+$ echo 0000:01:00.0 > /sys/bus/pci/drivers/ixgbe/bind
+$ ip link set ens1f0 addr 02:00:00:00:00:00
+$ ethtool --set-channels ens1f0 combined 1
+```

src/apps/xdp/bpf.lua

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+-- Use of this source code is governed by the Apache 2.0 license; see COPYING.
+
+module(...,package.seeall)
+
+local ffi = require("ffi")
+local bor = bit.bor
+
+-- BPF: just enough eBPF to assemble trivial XDP programs.
+--
+-- See "BPF Architecture":
+--   https://docs.cilium.io/en/v1.6/bpf/#bpf-architecture
+--
+-- See Linux v4.19:
+--   include/uapi/linux/bpf_common.h
+--   include/uapi/linux/bpf.h
+--   tools/include/linux/filter.h
+
+ins = ffi.typeof[[
+   struct {
+      uint8_t op;      /* opcode */
+      uint8_t dst:4;   /* dest register */
+      uint8_t src:4;   /* source register */
+      int16_t off;     /* signed offset */
+      int32_t imm;     /* signed immediate constant */
+   } __attribute__((packed))
+]]
+
+c = { -- Op class
+   LD = 0x00,
+   LDX = 0x01,
+   ST = 0x02,
+   STX = 0x03,
+   ALU = 0x04,
+   JMP = 0x05,
+   RET = 0x06,
+   MISC = 0x07,
+   ALU64 = 0x07 -- alu mode in double word width
+}
+
+f = { -- Load/store width
+   W  = 0x00,  -- 32-bit
+   H  = 0x08,  -- 16-bit
+   B  = 0x10,  -- 8-bit
+   DW = 0x18   -- 64-bit
+}
+
+m = { -- Op mode
+   IMM = 0x00,
+   ABS = 0x20,
+   IND = 0x40,
+   MEM = 0x60,
+   LEN = 0x80,
+   MSH = 0xa0,
+   XADD = 0xc0 -- exclusive add
+}
+
+a = { -- ALU mode
+   ADD = 0x00,
+   SUB = 0x10,
+   MUL = 0x20,
+   DIV = 0x30,
+   OR = 0x40,
+   AND = 0x50,
+   LSH = 0x60,
+   RSH = 0x70,
+   NEG = 0x80,
+   MOD = 0x90,
+   XOR = 0xa0,
+   MOV = 0xb0
+}
+
+s = { -- Src mode
+   K = 0x00,
+   X = 0x08,
+   MAP_FD = 0x01
+}
+
+j = { -- JMP mode
+   JA   = 0x00,
+   JEQ  = 0x10,
+   JGT  = 0x20,
+   JGE  = 0x30,
+   JSET = 0x40,
+   JNE  = 0x50,
+   JLT  = 0xa0,
+   JLE  = 0xb0,
+   JSGT = 0x60,
+   JSGE = 0x70,
+   JSLT = 0xc0,
+   JSLE = 0xd0,
+   CALL = 0x80,
+   EXIT = 0x90
+}
+
+fn = { -- Built-in helpers
+   unspec = 0,
+   map_lookup_elem = 1,
+   map_update_elem = 2,
+   map_delete_elem = 3,
+   probe_read = 4,
+   ktime_get_ns = 5,
+   trace_printk = 6,
+   get_prandom_u32 = 7,
+   get_smp_processor_id = 8,
+   skb_store_bytes = 9,
+   l3_csum_replace = 10,
+   l4_csum_replace = 11,
+   tail_call = 12,
+   clone_redirect = 13,
+   get_current_pid_tgid = 14,
+   get_current_uid_gid = 15,
+   get_current_comm = 16,
+   get_cgroup_classid = 17,
+   skb_vlan_push = 18,
+   skb_vlan_pop = 19,
+   skb_get_tunnel_key = 20,
+   skb_set_tunnel_key = 21,
+   perf_event_read = 22,
+   redirect = 23,
+   get_route_realm = 24,
+   perf_event_output = 25,
+   skb_load_bytes = 26,
+   get_stackid = 27,
+   csum_diff = 28,
+   skb_get_tunnel_opt = 29,
+   skb_set_tunnel_opt = 30,
+   skb_change_proto = 31,
+   skb_change_type = 32,
+   skb_under_cgroup = 33,
+   get_hash_recalc = 34,
+   get_current_task = 35,
+   probe_write_user = 36,
+   current_task_under_cgroup = 37,
+   skb_change_tail = 38,
+   skb_pull_data = 39,
+   csum_update = 40,
+   set_hash_invalid = 41,
+   get_numa_node_id = 42,
+   skb_change_head = 43,
+   xdp_adjust_head = 44,
+   probe_read_str = 45,
+   get_socket_cookie = 46,
+   get_socket_uid = 47,
+   set_hash = 48,
+   setsockopt = 49,
+   skb_adjust_room = 50,
+   redirect_map = 51,
+   sk_redirect_map = 52,
+   sock_map_update = 53,
+   xdp_adjust_meta = 54,
+   perf_event_read_value = 55,
+   perf_prog_read_value = 56,
+   getsockopt = 57,
+   override_return = 58,
+   sock_ops_cb_flags_set = 59,
+   msg_redirect_map = 60,
+   msg_apply_bytes = 61,
+   msg_cork_bytes = 62,
+   msg_pull_data = 63,
+   bind = 64,
+   xdp_adjust_tail = 65,
+   skb_get_xfrm_state = 66,
+   get_stack = 67,
+   skb_load_bytes_relative = 68,
+   fib_lookup = 69,
+   sock_hash_update = 70,
+   msg_redirect_hash = 71,
+   sk_redirect_hash = 72,
+   lwt_push_encap = 73,
+   lwt_seg6_store_bytes = 74,
+   lwt_seg6_adjust_srh = 75,
+   lwt_seg6_action = 76,
+   rc_repeat = 77,
+   rc_keydown = 78,
+   skb_cgroup_id = 79,
+   get_current_cgroup_id = 80,
+   get_local_storage = 81,
+   sk_select_reuseport = 82,
+   skb_ancestor_cgroup_id = 83,
+}
+
+function asm (insn) return ffi.typeof("$[?]", ins)(#insn, insn) end