udp gso/gro: move policy to client; send explicit params; server accepts and applies locally

Client remains the source of truth for UDP GSO/GRO policy (including --no-gsro). During parameter exchange, the client now sends GSO/GRO flags and sizes in JSON, and the server simply consumes those values without recomputing.

Kernel capability gating stays local and authoritative: each endpoint attempts to enable GSO/GRO on its own sockets via setsockopt, and if the kernel rejects it, we log and flip the local flag off. This handles the case where only one side supports the feature (GSO for the sender, GRO for the receiver).

Backward compatibility: if talking to an older client that doesn’t send GSO fields, the server derives gso_dg_size from blksize and adjusts gso_bf_size, falling back to DEFAULT_UDP_BLKSIZE if zero. This preserves previous behavior without overriding explicit client intent.

Behavior details:

- --no-gsro on the client sends gso=0 and gro=0, so the server won’t try to enable them.

- If -l/--length is provided, blksize (and therefore gso_dg_size when enabled) follows that value; otherwise default logic applies.

Files: src/iperf_api.c (send_parameters adds gso/gro fields; get_parameters reads them and removes server-side recompute unless needed for compatibility).

Author: gegles

src/iperf_api.c

@@ -2479,6 +2479,22 @@ send_parameters(struct iperf_test *test)
 	    cJSON_AddNumberToObject(j, "pacing_timer", test->settings->pacing_timer);
 	if (test->settings->burst)
 	    cJSON_AddNumberToObject(j, "burst", test->settings->burst);
+
+#ifdef HAVE_UDP_SEGMENT
+	/* Send UDP GSO settings from client to server */
+	if (test->protocol->id == Pudp) {
+	    cJSON_AddNumberToObject(j, "gso", test->settings->gso);
+	    cJSON_AddNumberToObject(j, "gso_dg_size", test->settings->gso_dg_size);
+	    cJSON_AddNumberToObject(j, "gso_bf_size", test->settings->gso_bf_size);
+	}
+#endif
+#ifdef HAVE_UDP_GRO
+	/* Send UDP GRO settings from client to server */
+	if (test->protocol->id == Pudp) {
+	    cJSON_AddNumberToObject(j, "gro", test->settings->gro);
+	    cJSON_AddNumberToObject(j, "gro_bf_size", test->settings->gro_bf_size);
+	}
+#endif
 	if (test->settings->tos)
 	    cJSON_AddNumberToObject(j, "TOS", test->settings->tos);
 	if (test->settings->flowlabel)
@@ -2602,17 +2618,32 @@ get_parameters(struct iperf_test *test)
 	    test->settings->socket_bufsize = j_p->valueint;
 	if ((j_p = iperf_cJSON_GetObjectItemType(j, "len", cJSON_Number)) != NULL)
 	    test->settings->blksize = j_p->valueint;
+
 #ifdef HAVE_UDP_SEGMENT
-	if (test->protocol->id == Pudp && test->settings->gso == 1) {
+	/* Accept UDP GSO settings provided by the client */
+	if ((j_p = iperf_cJSON_GetObjectItemType(j, "gso", cJSON_Number)) != NULL)
+	    test->settings->gso = j_p->valueint;
+	if ((j_p = iperf_cJSON_GetObjectItemType(j, "gso_dg_size", cJSON_Number)) != NULL)
+	    test->settings->gso_dg_size = j_p->valueint;
+	if ((j_p = iperf_cJSON_GetObjectItemType(j, "gso_bf_size", cJSON_Number)) != NULL)
+	    test->settings->gso_bf_size = j_p->valueint;
+
+	/* Backward-compatibility: If client didn't send GSO params, derive from blksize. */
+	if (test->protocol->id == Pudp && test->settings->gso == 1 && test->settings->gso_dg_size == 0) {
 	    test->settings->gso_dg_size = test->settings->blksize;
-	    /* use the multiple of datagram size for the best efficiency. */
 	    if (test->settings->gso_dg_size > 0) {
 	        test->settings->gso_bf_size = (test->settings->gso_bf_size / test->settings->gso_dg_size) * test->settings->gso_dg_size;
 	    } else {
-	        /* If gso_dg_size is 0 (unlimited bandwidth), use default UDP datagram size */
 	        test->settings->gso_dg_size = DEFAULT_UDP_BLKSIZE;
 	    }
 	}
+#endif
+#ifdef HAVE_UDP_GRO
+	/* Accept UDP GRO settings provided by the client */
+	if ((j_p = iperf_cJSON_GetObjectItemType(j, "gro", cJSON_Number)) != NULL)
+	    test->settings->gro = j_p->valueint;
+	if ((j_p = iperf_cJSON_GetObjectItemType(j, "gro_bf_size", cJSON_Number)) != NULL)
+	    test->settings->gro_bf_size = j_p->valueint;
 #endif
 	if ((j_p = iperf_cJSON_GetObjectItemType(j, "bandwidth", cJSON_Number)) != NULL)
 	    test->settings->rate = j_p->valueint;