tcp: Introduce scatter-gather IO path from socket to tap

...similarly to what was done for UDP. Quick performance test with
32KiB buffers, host to VM:

$ iperf3 -c 192.0.2.2 -N
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec  8.47 GBytes  7.27 Gbits/sec    0             sender
[  5]   0.00-10.00  sec  8.45 GBytes  7.26 Gbits/sec                  receiver

$ iperf3 -c 2a01:598:88ba:a056:271f:473a:c0d9:abc1
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec  8.43 GBytes  7.24 Gbits/sec    0             sender
[  5]   0.00-10.00  sec  8.41 GBytes  7.22 Gbits/sec                  receiver

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/util.h b/util.h
index 90d7f7c..221df97 100644
--- a/util.h
+++ b/util.h
@@ -29,6 +29,8 @@ void debug(const char *format, ...);
 #define MAX(x, y)		(((x) > (y)) ? (x) : (y))
 #endif
 
+#define DIV_ROUND_UP(n, d)	(((n) + (d) - 1) / (d))
+
 #define SWAP(a, b)							\
 	do {								\
 		typeof(a) __x = (a); (a) = (b); (b) = __x;		\