[PATCH] tcp: Add missing EPOLLET flag for established sockets

[Asahi Lina <lina@asahilina.net>]

When there is unacknowledged data in the inbound socket buffer, passt
leaves the socket in the epoll instance to accept new data from the
server. Since there is already data in the socket buffer, an epoll
without EPOLLET will repeatedly fire while no data is processed,
busy-looping the CPU:

epoll_pwait(3, [...], 8, 1000, NULL, 8) = 4
recvmsg(25, {msg_namelen=0}, MSG_PEEK)  = -1 EAGAIN (Resource temporarily unavailable)
recvmsg(169, {msg_namelen=0}, MSG_PEEK) = -1 EAGAIN (Resource temporarily unavailable)
recvmsg(111, {msg_namelen=0}, MSG_PEEK) = -1 EAGAIN (Resource temporarily unavailable)
recvmsg(180, {msg_namelen=0}, MSG_PEEK) = -1 EAGAIN (Resource temporarily unavailable)
epoll_pwait(3, [...], 8, 1000, NULL, 8) = 4
recvmsg(25, {msg_namelen=0}, MSG_PEEK)  = -1 EAGAIN (Resource temporarily unavailable)
recvmsg(169, {msg_namelen=0}, MSG_PEEK) = -1 EAGAIN (Resource temporarily unavailable)
recvmsg(111, {msg_namelen=0}, MSG_PEEK) = -1 EAGAIN (Resource temporarily unavailable)
recvmsg(180, {msg_namelen=0}, MSG_PEEK) = -1 EAGAIN (Resource temporarily unavailable)

Add in the missing EPOLLET flag for this case. This brings CPU
usage down from around ~80% when downloading over TCP, to ~5% (use
case: passt as network transport for muvm, downloading Steam games).

Signed-off-by: Asahi Lina <lina@asahilina.net>
---
 tcp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tcp.c b/tcp.c
index ec433f7d54bcccc4f1ba33e7add10c1e61807bc8..38042264a83570145d7fe9577d9fe1928435ff4d 100644
--- a/tcp.c
+++ b/tcp.c
@@ -439,7 +439,7 @@ static uint32_t tcp_conn_epoll_events(uint8_t events, uint8_t conn_flags)
 		if (conn_flags & STALLED)
 			return EPOLLIN | EPOLLOUT | EPOLLRDHUP | EPOLLET;
 
-		return EPOLLIN | EPOLLRDHUP;
+		return EPOLLIN | EPOLLRDHUP | EPOLLET;
 	}
 
 	if (events == TAP_SYN_RCVD)

---
base-commit: e5ba8adef71ec53e192373ed1267dc338719dda0
change-id: 20241228-tcp-epollet-fix-3f8e9c736cd1

Cheers,
~~ Lina