Google cloud routes traffic using IP addresses without the support of MAC
addresses, so changing source IP address for txonly-multi-flow can have
negative performance implications for net/gve when using testpmd. This
patch updates txonly multiflow mode to modify source ports instead of
source IP addresses.

The change can be tested with the following command:
dpdk-testpmd -- --forward-mode=txonly --txonly-multi-flow \
    --tx-ip=<SRC>,<DST>

Signed-off-by: Joshua Washington <joshwash@google.com>
Reviewed-by: Rushil Gupta <rushilg@google.com>
---
 app/test-pmd/txonly.c | 39 +++++++++++++++++++++++----------------
 1 file changed, 23 insertions(+), 16 deletions(-)

diff --git a/app/test-pmd/txonly.c b/app/test-pmd/txonly.c
index b3d6873104..f79e0e5d0b 100644
--- a/app/test-pmd/txonly.c
+++ b/app/test-pmd/txonly.c
@@ -56,7 +56,7 @@ uint32_t tx_ip_dst_addr = (198U << 24) | (18 << 16) | (0 << 8) | 2;
 #define IP_DEFTTL  64   /* from RFC 1340. */

 static struct rte_ipv4_hdr pkt_ip_hdr; /**< IP header of transmitted packets. */
-RTE_DEFINE_PER_LCORE(uint8_t, _ip_var); /**< IP address variation */
+RTE_DEFINE_PER_LCORE(uint8_t, _src_var); /**< Source port variation */
 static struct rte_udp_hdr pkt_udp_hdr; /**< UDP header of tx packets. */

 static uint64_t timestamp_mask; /**< Timestamp dynamic flag mask */
@@ -230,28 +230,35 @@ pkt_burst_prepare(struct rte_mbuf *pkt, struct rte_mempool *mbp,
        copy_buf_to_pkt(eth_hdr, sizeof(*eth_hdr), pkt, 0);
        copy_buf_to_pkt(&pkt_ip_hdr, sizeof(pkt_ip_hdr), pkt,
                        sizeof(struct rte_ether_hdr));
+       copy_buf_to_pkt(&pkt_udp_hdr, sizeof(pkt_udp_hdr), pkt,
+                       sizeof(struct rte_ether_hdr) +
+                       sizeof(struct rte_ipv4_hdr));
        if (txonly_multi_flow) {
-               uint8_t  ip_var = RTE_PER_LCORE(_ip_var);
-               struct rte_ipv4_hdr *ip_hdr;
-               uint32_t addr;
+               uint16_t src_var = RTE_PER_LCORE(_src_var);
+               struct rte_udp_hdr *udp_hdr;
+               uint16_t port;

-               ip_hdr = rte_pktmbuf_mtod_offset(pkt,
-                               struct rte_ipv4_hdr *,
-                               sizeof(struct rte_ether_hdr));
+               udp_hdr = rte_pktmbuf_mtod_offset(pkt,
+                               struct rte_udp_hdr *,
+                               sizeof(struct rte_ether_hdr) +
+                               sizeof(struct rte_ipv4_hdr));
                /*
-                * Generate multiple flows by varying IP src addr. This
-                * enables packets are well distributed by RSS in
+                * Generate multiple flows by varying UDP source port.
+                * This enables packets are well distributed by RSS in
                 * receiver side if any and txonly mode can be a decent
                 * packet generator for developer's quick performance
                 * regression test.
+                *
+                * Only ports in the range 49152 (0xC000) and 65535 (0xFFFF)
+                * will be used, with the least significant byte representing
+                * the lcore ID. As such, the most significant byte will cycle
+                * through 0xC0 and 0xFF.
                 */
-               addr = (tx_ip_dst_addr | (ip_var++ << 8)) + rte_lcore_id();
-               ip_hdr->src_addr = rte_cpu_to_be_32(addr);
-               RTE_PER_LCORE(_ip_var) = ip_var;
+               port = ((((src_var++) % (0xFF - 0xC0) + 0xC0) & 0xFF) << 8)
+                               + rte_lcore_id();
+               udp_hdr->src_port = rte_cpu_to_be_16(port);
+               RTE_PER_LCORE(_src_var) = src_var;
        }
-       copy_buf_to_pkt(&pkt_udp_hdr, sizeof(pkt_udp_hdr), pkt,
-                       sizeof(struct rte_ether_hdr) +
-                       sizeof(struct rte_ipv4_hdr));

        if (unlikely(tx_pkt_split == TX_PKT_SPLIT_RND) || txonly_multi_flow)
                update_pkt_header(pkt, pkt_len);
@@ -393,7 +400,7 @@ pkt_burst_transmit(struct fwd_stream *fs)
        nb_tx = common_fwd_stream_transmit(fs, pkts_burst, nb_pkt);

        if (txonly_multi_flow)
-               RTE_PER_LCORE(_ip_var) -= nb_pkt - nb_tx;
+               RTE_PER_LCORE(_src_var) -= nb_pkt - nb_tx;

        if (unlikely(nb_tx < nb_pkt)) {
                if (verbose_level > 0 && fs->fwd_dropped == 0)
--
2.40.0.634.g4ca3ef3211-goog