[PATCH bpf-next] libbpf: remove explicit XSKMAP lookup from AF_XDP XDP program

[Björn Töpel]

From: Björn Töpel <bjorn.to...@intel.com>

In commit 43e74c0267a3 ("bpf_xdp_redirect_map: Perform map lookup in
eBPF helper") the bpf_redirect_map() helper learned to do map lookup,
which means that the explicit lookup in the XDP program for AF_XDP is
not needed.

This commit removes the map lookup, which simplifies the BPF code and
improves the performance for the "rx_drop" [1] scenario with ~4%.

[1] # xdpsock -i eth0 -z -r

Suggested-by: Toke Høiland-Jørgensen <t...@redhat.com>
Signed-off-by: Björn Töpel <bjorn.to...@intel.com>
---
 tools/lib/bpf/xsk.c | 24 +++++-------------------
 1 file changed, 5 insertions(+), 19 deletions(-)

diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
index b0f532544c91..8e35de3cb443 100644
--- a/tools/lib/bpf/xsk.c
+++ b/tools/lib/bpf/xsk.c
@@ -278,29 +278,15 @@ static int xsk_load_xdp_prog(struct xsk_socket *xsk)
         *
         *     // A set entry here means that the correspnding queue_id
         *     // has an active AF_XDP socket bound to it.
-        *     if (bpf_map_lookup_elem(&xsks_map, &index))
-        *         return bpf_redirect_map(&xsks_map, index, 0);
-        *
-        *     return XDP_PASS;
+        *     return bpf_redirect_map(&xsks_map, index, XDP_PASS);
         * }
         */
        struct bpf_insn prog[] = {
-               /* r1 = *(u32 *)(r1 + 16) */
-               BPF_LDX_MEM(BPF_W, BPF_REG_1, BPF_REG_1, 16),
-               /* *(u32 *)(r10 - 4) = r1 */
-               BPF_STX_MEM(BPF_W, BPF_REG_10, BPF_REG_1, -4),
-               BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
-               BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -4),
-               BPF_LD_MAP_FD(BPF_REG_1, xsk->xsks_map_fd),
-               BPF_EMIT_CALL(BPF_FUNC_map_lookup_elem),
-               BPF_MOV64_REG(BPF_REG_1, BPF_REG_0),
-               BPF_MOV32_IMM(BPF_REG_0, 2),
-               /* if r1 == 0 goto +5 */
-               BPF_JMP_IMM(BPF_JEQ, BPF_REG_1, 0, 5),
-               /* r2 = *(u32 *)(r10 - 4) */
+               /* r2 = *(u32 *)(r1 + 16) */
+               BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1, 16),
                BPF_LD_MAP_FD(BPF_REG_1, xsk->xsks_map_fd),
-               BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_10, -4),
-               BPF_MOV32_IMM(BPF_REG_3, 0),
+               /* r3 = XDP_PASS */
+               BPF_MOV32_IMM(BPF_REG_3, 2),
                BPF_EMIT_CALL(BPF_FUNC_redirect_map),
                /* The jumps are to this instruction */
                BPF_EXIT_INSN(),
-- 
2.20.1