dp-packet: Avoid checks while preparing non-offloading packets.

Currently, dp_packet_ol_send_prepare() performs multiple checks for
each offloading flag separately.  That takes a noticeable amount of
extra cycles for packets that do not have any offloading flags set.

Skip most of the work if no checksumming flags are set.

The change improves performance of direct forwarding between two
virtio-user ports (V2V) by ~2.5 % and offsets all the negative
effects of TSO support introduced recently.

It adds an extra check to the offloading path, but it is not a
default configuration and also should take much smaller hit due
to lower number of larger packets.

Acked-by: Mike Pattrick <mkp@redhat.com>
Acked-by: Simon Horman <horms@ovn.org>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

lib/dp-packet.c

@@ -576,6 +576,11 @@ dp_packet_ol_send_prepare(struct dp_packet *p, uint64_t flags)
 {
     bool tnl_inner = false;
 
+    if (!dp_packet_hwol_tx_is_any_csum(p)) {
+        /* Only checksumming needs actions. */
+        return;
+    }
+
     if (dp_packet_hwol_is_tunnel_geneve(p) ||
         dp_packet_hwol_is_tunnel_vxlan(p)) {
         tnl_inner = true;

lib/dp-packet.h

@@ -131,6 +131,10 @@ enum dp_packet_offload_mask {
 #define DP_PACKET_OL_TX_L4_MASK (DP_PACKET_OL_TX_TCP_CKSUM | \
                                  DP_PACKET_OL_TX_UDP_CKSUM | \
                                  DP_PACKET_OL_TX_SCTP_CKSUM)
+#define DP_PACKET_OL_TX_ANY_CKSUM (DP_PACKET_OL_TX_L4_MASK | \
+                                   DP_PACKET_OL_TX_IP_CKSUM | \
+                                   DP_PACKET_OL_TX_OUTER_IP_CKSUM | \
+                                   DP_PACKET_OL_TX_OUTER_UDP_CKSUM)
 #define DP_PACKET_OL_RX_IP_CKSUM_MASK (DP_PACKET_OL_RX_IP_CKSUM_GOOD | \
                                        DP_PACKET_OL_RX_IP_CKSUM_BAD)
 #define DP_PACKET_OL_RX_L4_CKSUM_MASK (DP_PACKET_OL_RX_L4_CKSUM_GOOD | \
@@ -1189,6 +1193,13 @@ dp_packet_hwol_is_outer_udp_cksum(struct dp_packet *b)
     return !!(*dp_packet_ol_flags_ptr(b) & DP_PACKET_OL_TX_OUTER_UDP_CKSUM);
 }
 
+/* Returns 'true' if packet 'b' is marked for any checksum offload. */
+static inline bool
+dp_packet_hwol_tx_is_any_csum(struct dp_packet *b)
+{
+    return !!(*dp_packet_ol_flags_ptr(b) & DP_PACKET_OL_TX_ANY_CKSUM);
+}
+
 static inline void
 dp_packet_hwol_reset_tx_l4_csum(struct dp_packet *p)
 {