[PATCH v2 05/10] tcp: On socket EPOLLOUT, send new ACK to tap immediately

[David Gibson <david@gibson.dropbear.id.au>]

When the guest sends data on a TCP connection, it's possible that we run
out of buffer space on the socket side.  If that happens we'll advertise
a zero window to the guest stopping it from sending.  When the socket side
buffer clears again, which is signalled by an EPOLLOUT, we recalculate the
ack and window with tcp_update_seqack_wnd().

tcp_update_seqack_wnd() only calculates the new values, it doesn't actually
send out the ack - that will typically happen some milliseconds later on
the ACK_TO_TAP_DUE timer.

AFAICT, there's not really any point delaying this ack though, we might as
well send it and get the guest sending again ASAP.  So, instead of
calling tcp_update_seqack_wnd() call tcp_send_flag() to send an immediate
ACK if needed.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 tcp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tcp.c b/tcp.c
index ee894c5c..32e45e09 100644
--- a/tcp.c
+++ b/tcp.c
@@ -2240,7 +2240,7 @@ void tcp_sock_handler(const struct ctx *c, union epoll_ref ref,
 			tcp_data_from_sock(c, conn);
 
 		if (events & EPOLLOUT)
-			tcp_update_seqack_wnd(c, conn, false, NULL);
+			tcp_send_flag(c, conn, ACK_IF_NEEDED);
 
 		return;
 	}
-- 
@@ -2240,7 +2240,7 @@ void tcp_sock_handler(const struct ctx *c, union epoll_ref ref,
 			tcp_data_from_sock(c, conn);
 
 		if (events & EPOLLOUT)
-			tcp_update_seqack_wnd(c, conn, false, NULL);
+			tcp_send_flag(c, conn, ACK_IF_NEEDED);
 
 		return;
 	}
-- 
2.46.0