Commit d7d962a0 authored by Björn Töpel's avatar Björn Töpel Committed by Alexei Starovoitov
Browse files

libbpf: Use implicit XSKMAP lookup from AF_XDP XDP program 

In commit 43e74c02 ("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 for post-5.3 kernels.

This commit adds the implicit map lookup with default action, which
improves the performance for the "rx_drop" [1] scenario with ~4%.

For pre-5.3 kernels, the bpf_redirect_map() returns XDP_ABORTED, and a
fallback path for backward compatibility is entered, where explicit
lookup is still performed. This means a slight regression for older
kernels (an additional bpf_redirect_map() call), but I consider that a
fair punishment for users not upgrading their kernels. ;-)

v1->v2: Backward compatibility (Toke) [2]
v2->v3: Avoid masking/zero-extension by using JMP32 [3]

[1] # xdpsock -i eth0 -z -r
[2] https://lore.kernel.org/bpf/87pnirb3dc.fsf@toke.dk/
[3] https://lore.kernel.org/bpf/87v9sip0i8.fsf@toke.dk/



Suggested-by: default avatarToke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: default avatarBjörn Töpel <bjorn.topel@intel.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Acked-by: default avatarToke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20191022072206.6318-1-bjorn.topel@gmail.com
parent 5e5b03d1

tools/lib/bpf/xsk.c

+32 −10
Original line number Diff line number Diff line
@@ -274,33 +274,55 @@ static int xsk_load_xdp_prog(struct xsk_socket *xsk) 
	/* This is the C-program:

	 * SEC("xdp_sock") int xdp_sock_prog(struct xdp_md *ctx)

	 * {

	 *     int index = ctx->rx_queue_index;

	 *     int ret, index = ctx->rx_queue_index;

	 *

	 *     // A set entry here means that the correspnding queue_id

	 *     // has an active AF_XDP socket bound to it.

	 *     ret = bpf_redirect_map(&xsks_map, index, XDP_PASS);

	 *     if (ret > 0)

	 *         return ret;

	 *

	 *     // Fallback for pre-5.3 kernels, not supporting default

	 *     // action in the flags parameter.

	 *     if (bpf_map_lookup_elem(&xsks_map, &index))

	 *         return bpf_redirect_map(&xsks_map, index, 0);

	 *

	 *     return 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),

		/* r2 = *(u32 *)(r1 + 16) */

		BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1, 16),

		/* *(u32 *)(r10 - 4) = r2 */

		BPF_STX_MEM(BPF_W, BPF_REG_10, BPF_REG_2, -4),

		/* r1 = xskmap[] */

		BPF_LD_MAP_FD(BPF_REG_1, xsk->xsks_map_fd),

		/* r3 = XDP_PASS */

		BPF_MOV64_IMM(BPF_REG_3, 2),

		/* call bpf_redirect_map */

		BPF_EMIT_CALL(BPF_FUNC_redirect_map),

		/* if w0 != 0 goto pc+13 */

		BPF_JMP32_IMM(BPF_JSGT, BPF_REG_0, 0, 13),

		/* r2 = r10 */

		BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),

		/* r2 += -4 */

		BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -4),

		/* r1 = xskmap[] */

		BPF_LD_MAP_FD(BPF_REG_1, xsk->xsks_map_fd),

		/* call bpf_map_lookup_elem */

		BPF_EMIT_CALL(BPF_FUNC_map_lookup_elem),

		/* r1 = r0 */

		BPF_MOV64_REG(BPF_REG_1, BPF_REG_0),

		BPF_MOV32_IMM(BPF_REG_0, 2),

		/* if r1 == 0 goto +5 */

		/* r0 = XDP_PASS */

		BPF_MOV64_IMM(BPF_REG_0, 2),

		/* if r1 == 0 goto pc+5 */

		BPF_JMP_IMM(BPF_JEQ, BPF_REG_1, 0, 5),

		/* r2 = *(u32 *)(r10 - 4) */

		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),

		/* r1 = xskmap[] */

		BPF_LD_MAP_FD(BPF_REG_1, xsk->xsks_map_fd),

		/* r3 = 0 */

		BPF_MOV64_IMM(BPF_REG_3, 0),

		/* call bpf_redirect_map */

		BPF_EMIT_CALL(BPF_FUNC_redirect_map),

		/* The jumps are to this instruction */

		BPF_EXIT_INSN(),

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