passt: Spare some syscalls, add some optimisations from profiling

Avoid a bunch of syscalls on forwarding paths by:

- storing minimum and maximum file descriptor numbers for each
  protocol, fall back to SO_PROTOCOL query only on overlaps

- allocating a larger receive buffer -- this can result in more
  coalesced packets than sendmmsg() can take (UIO_MAXIOV, i.e. 1024),
  so make sure we don't exceed that within a single call to protocol
  tap handlers

- nesting the handling loop in tap_handler() in the receive loop,
  so that we have better chances of filling our receive buffer in
  fewer calls

- skipping the recvfrom() in the UDP handler on EPOLLERR -- there's
  nothing to be done in that case

and while at it:

- restore the 20ms timer interval for periodic (TCP) events, I
  accidentally changed that to 100ms in an earlier commit

- attempt using SO_ZEROCOPY for UDP -- if it's not available,
  sendmmsg() will succeed anyway

- fix the handling of the status code from sendmmsg(), if it fails,
  we'll try to discard the first message, hence return 1 from the
  UDP handler

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

1 files changed, 2 insertions, 2 deletions

diff --git a/tap.c b/tap.c
index f8b8b4f..c11191c 100644
--- a/tap.c
+++ b/tap.c
@@ -37,9 +37,9 @@
 int tap_send(int fd, void *data, size_t len, int flags)
 {
 	uint32_t vnet_len = htonl(len);
-	send(fd, &vnet_len, 4, 0);
+	send(fd, &vnet_len, 4, MSG_DONTWAIT | MSG_NOSIGNAL);
 
-	return send(fd, data, len, flags);
+	return send(fd, data, len, flags | MSG_DONTWAIT | MSG_NOSIGNAL);
 }
 
 /**