Re: [PATCH 3/6] tcp: Rewind sequence when guest shrinks window to zero

On Fri, Aug 15, 2025 at 06:10:39PM +0200, Stefano Brivio wrote:

A window shrunk to zero means by definition that anything else that might be in flight is now out of window. Restart from the currently acknowledged sequence.

Suggested-by: Jon Maloy Signed-off-by: Stefano Brivio

Reviewed-by: David Gibson

--- tcp.c | 25 ++++++++++++++++--------- 1 file changed, 16 insertions(+), 9 deletions(-)

diff --git a/tcp.c b/tcp.c index 1402ca2..dda0a2e 100644 --- a/tcp.c +++ b/tcp.c @@ -1257,19 +1257,25 @@ static void tcp_get_tap_ws(struct tcp_tap_conn *conn,

/** * tcp_tap_window_update() - Process an updated window from tap side + * @c: Execution context * @conn: Connection pointer * @wnd: Window value, host order, unscaled */ -static void tcp_tap_window_update(struct tcp_tap_conn *conn, unsigned wnd) +static void tcp_tap_window_update(const struct ctx *c, + struct tcp_tap_conn *conn, unsigned wnd) { wnd = MIN(MAX_WINDOW, wnd << conn->ws_from_tap);

/* Work-around for bug introduced in peer kernel code, commit - * e2142825c120 ("net: tcp: send zero-window ACK when no memory"). - * We don't update if window shrank to zero. + * e2142825c120 ("net: tcp: send zero-window ACK when no memory"): don't + * update the window if it shrank to zero, so that we'll eventually + * retry to send data, but rewind the sequence as that obviously implies + * that no data beyond the updated window will ever be acknowledged. */ - if (!wnd && SEQ_LT(conn->seq_ack_from_tap, conn->seq_to_tap)) + if (!wnd && SEQ_LT(conn->seq_ack_from_tap, conn->seq_to_tap)) { + tcp_rewind_seq(c, conn); return; + }

conn->wnd_from_tap = MIN(wnd >> conn->ws_from_tap, USHRT_MAX);

@@ -1694,7 +1700,8 @@ static int tcp_data_from_tap(const struct ctx *c, struct tcp_tap_conn *conn, tcp_timer_ctl(c, conn);

if (p->count == 1) { - tcp_tap_window_update(conn, ntohs(th->window)); + tcp_tap_window_update(c, conn, + ntohs(th->window)); return 1; }

@@ -1772,7 +1779,7 @@ static int tcp_data_from_tap(const struct ctx *c, struct tcp_tap_conn *conn, if (ack && !tcp_sock_consume(conn, max_ack_seq)) tcp_update_seqack_from_tap(c, conn, max_ack_seq);

- tcp_tap_window_update(conn, max_ack_seq_wnd); + tcp_tap_window_update(c, conn, max_ack_seq_wnd);

if (retr) { flow_trace(conn, @@ -1861,7 +1868,7 @@ static void tcp_conn_from_sock_finish(const struct ctx *c, const struct tcphdr *th, const char *opts, size_t optlen) { - tcp_tap_window_update(conn, ntohs(th->window)); + tcp_tap_window_update(c, conn, ntohs(th->window)); tcp_get_tap_ws(conn, opts, optlen);

/* First value is not scaled */ @@ -2059,7 +2066,7 @@ int tcp_tap_handler(const struct ctx *c, uint8_t pif, sa_family_t af, if (!th->ack) goto reset;

- tcp_tap_window_update(conn, ntohs(th->window)); + tcp_tap_window_update(c, conn, ntohs(th->window));

tcp_data_from_sock(c, conn);

@@ -2071,7 +2078,7 @@ int tcp_tap_handler(const struct ctx *c, uint8_t pif, sa_family_t af, if (conn->events & TAP_FIN_RCVD) { tcp_sock_consume(conn, ntohl(th->ack_seq)); tcp_update_seqack_from_tap(c, conn, ntohl(th->ack_seq)); - tcp_tap_window_update(conn, ntohs(th->window)); + tcp_tap_window_update(c, conn, ntohs(th->window)); tcp_data_from_sock(c, conn);

if (conn->events & SOCK_FIN_RCVD &&

-- David Gibson (he or they) | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you, not the other way | around. http://www.ozlabs.org/~dgibson