Re: [PATCH v3 3/7] tcp: Rewind sequence when guest shrinks window to zero

On Mon, 1 Sep 2025 14:28:33 +1000 David Gibson wrote:

On Fri, Aug 29, 2025 at 10:11:28PM +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.

We need to do that both in tcp_tap_window_update(), where we already check for zero-window updates, as well as in tcp_data_from_tap(), because we might get one of those updates in a batch of packets that also contains a non-zero window update.

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

Reviewed-by: David Gibson

...

--- tcp.c | 34 +++++++++++++++++++++++++--------- 1 file changed, 25 insertions(+), 9 deletions(-)

diff --git a/tcp.c b/tcp.c index 1402ca2..11c9c84 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.

Nit: Arguably "no data...will ever" is not quite right. It presumbly won't be acknowledged until we resend it at least once, but we certainly hope it will be acknowledged after that point.

Hmm, yes, it's "ever" in a rather relative sense (we'll time out, eventually, but that takes a while), which makes it not "ever", strictly speaking. Let me just drop "ever", unless you have better ideas on how to improve the comment. -- Stefano