xdp: add KNOT_XDP_LOAD_BPF_ALWAYS_UNLOAD mode

src/knot/server/server.c

@@ -234,6 +234,12 @@ static iface_t *server_init_xdp_iface(struct sockaddr_storage *addr, unsigned *t
 			(i == 0 ? KNOT_XDP_LOAD_BPF_ALWAYS : KNOT_XDP_LOAD_BPF_NEVER);
 		ret = knot_xdp_init(new_if->xdp_sockets + i, iface.name, i,
 		                    iface.port, mode);
+		if (ret == -EBUSY && i == 0) {
+			log_notice("XDP interface %s@%u is busy, retrying initializaion",
+			           iface.name, iface.port);
+			ret = knot_xdp_init(new_if->xdp_sockets + i, iface.name, i,
+			                    iface.port, KNOT_XDP_LOAD_BPF_ALWAYS_UNLOAD);
+		}
 		if (ret != KNOT_EOK) {
 			log_warning("failed to initialize XDP interface %s@%u, queue %d (%s)",
 			            iface.name, iface.port, i, knot_strerror(ret));

src/libknot/xdp/bpf-user.c

@@ -238,6 +238,10 @@ int kxsk_iface_new(const char *if_name, int if_queue, knot_xdp_load_bpf_t load_b
 			}
 		}
 		break;
+	case KNOT_XDP_LOAD_BPF_ALWAYS_UNLOAD:
+		(void)bpf_set_link_xdp_fd(iface->if_index, -1, 0);
+		sleep(1);
+		// FALLTHROUGH
 	case KNOT_XDP_LOAD_BPF_ALWAYS:
 		ret = ensure_prog(iface, true);
 		break;

src/libknot/xdp/xdp.h

@@ -44,9 +44,10 @@ struct knot_xdp_msg {
  *       a compatible BPF program is loaded on the interface.
  */
 typedef enum {
-	KNOT_XDP_LOAD_BPF_NEVER,  /*!< Do not load; error out if not loaded already. */
-	KNOT_XDP_LOAD_BPF_ALWAYS, /*!< Always load a program (overwrite it). */
-	KNOT_XDP_LOAD_BPF_MAYBE,  /*!< Try with present program or load if none. */
+	KNOT_XDP_LOAD_BPF_NEVER,         /*!< Do not load; error out if not loaded already. */
+	KNOT_XDP_LOAD_BPF_ALWAYS,        /*!< Always load a program (overwrite it). */
+	KNOT_XDP_LOAD_BPF_ALWAYS_UNLOAD, /*!< KNOT_XDP_LOAD_BPF_ALWAYS + unload previous. */
+	KNOT_XDP_LOAD_BPF_MAYBE,         /*!< Try with present program or load if none. */
 	/* Implementation caveat: when re-using program in _MAYBE case, we get a message:
 	 * libbpf: Kernel error message: XDP program already attached */
 } knot_xdp_load_bpf_t;