From 518d8c1ef71bb9a6f38b4e83a0c97374882e4213 Mon Sep 17 00:00:00 2001 From: David Gibson Date: Wed, 8 Oct 2025 12:22:49 +1100 Subject: tcp: Clarify logic calculating how much guest data to ack This is fairly complex, because we have a method we prefer but we need to fall back to a simpler one in a bunch of cases. Slightly reorganise the code to make the flow clearer, and add a large comment giving the rationale. Signed-off-by: David Gibson Signed-off-by: Stefano Brivio --- tcp.c | 72 +++++++++++++++++++++++++++++++++++++++++++------------------------ 1 file changed, 46 insertions(+), 26 deletions(-) diff --git a/tcp.c b/tcp.c index 91aa330..0f9e9b3 100644 --- a/tcp.c +++ b/tcp.c @@ -1014,35 +1014,55 @@ int tcp_update_seqack_wnd(const struct ctx *c, struct tcp_tap_conn *conn, uint32_t new_wnd_to_tap = prev_wnd_to_tap; int s = conn->sock; - if (!bytes_acked_cap) { - conn->seq_ack_to_tap = conn->seq_from_tap; - if (SEQ_LT(conn->seq_ack_to_tap, prev_ack_to_tap)) - conn->seq_ack_to_tap = prev_ack_to_tap; - } else { - if ((unsigned)SNDBUF_GET(conn) < SNDBUF_SMALL || - tcp_rtt_dst_low(conn) || CONN_IS_CLOSING(conn) || - (conn->flags & LOCAL) || force_seq) { - conn->seq_ack_to_tap = conn->seq_from_tap; - } else if (conn->seq_ack_to_tap != conn->seq_from_tap) { - if (!tinfo) { - tinfo = &tinfo_new; - if (getsockopt(s, SOL_TCP, TCP_INFO, tinfo, &sl)) - return 0; - } - - /* This trips a cppcheck bug in some versions, including - * cppcheck 2.18.3. - * https://sourceforge.net/p/cppcheck/discussion/general/thread/fecde59085/ - */ - /* cppcheck-suppress [uninitvar,unmatchedSuppression] */ - conn->seq_ack_to_tap = tinfo->tcpi_bytes_acked + - conn->seq_init_from_tap; - - if (SEQ_LT(conn->seq_ack_to_tap, prev_ack_to_tap)) - conn->seq_ack_to_tap = prev_ack_to_tap; + /* At this point we could ack all the data we've accepted for forwarding + * (seq_from_tap). When possible, however, we want to only acknowledge + * what the peer has acknowledged. This makes it appear to the guest + * more like a direct connection to the peer, and may improve flow + * control behaviour. + * + * For it to be possible and worth it we need: + * - The TCP_INFO Linux extension which gives us the peer acked bytes + * - Not to be told not to (force_seq) + * - Not half-closed in the peer->guest direction + * With no data coming from the peer, we might not get events which + * would prompt us to recheck bytes_acked. We could poll on a + * timer, but that's more trouble than it's worth. + * - Not a host local connection + * Data goes from socket to socket, with nothing meaningfully "in + * flight". + * - Not a pseudo-local connection (e.g. to a VM on the same host) + * - Large enough send buffer + * In these cases, there's not enough in flight to bother. + */ + if (bytes_acked_cap && !force_seq && + !CONN_IS_CLOSING(conn) && + !(conn->flags & LOCAL) && !tcp_rtt_dst_low(conn) && + (unsigned)SNDBUF_GET(conn) >= SNDBUF_SMALL) { + if (!tinfo) { + tinfo = &tinfo_new; + if (getsockopt(s, SOL_TCP, TCP_INFO, tinfo, &sl)) + return 0; } + + /* This trips a cppcheck bug in some versions, including + * cppcheck 2.18.3. + * https://sourceforge.net/p/cppcheck/discussion/general/thread/fecde59085/ + */ + /* cppcheck-suppress [uninitvar,unmatchedSuppression] */ + conn->seq_ack_to_tap = tinfo->tcpi_bytes_acked + + conn->seq_init_from_tap; + } else { + /* Fall back to acknowledging everything we got */ + conn->seq_ack_to_tap = conn->seq_from_tap; } + /* It's occasionally possible for us to go from using the fallback above + * to the tcpi_bytes_acked method. In that case, we must be careful not + * to let our ACKed sequence go backwards. + */ + if (SEQ_LT(conn->seq_ack_to_tap, prev_ack_to_tap)) + conn->seq_ack_to_tap = prev_ack_to_tap; + if (!snd_wnd_cap) { tcp_get_sndbuf(conn); new_wnd_to_tap = MIN(SNDBUF_GET(conn), MAX_WINDOW); -- cgit v1.2.3