On Fri, May 17, 2024 at 11:24:12AM -0400, Jon Maloy wrote:
...
  commit a469fc393fa1 ("tcp, tap: Don't
increase tap-side sequence counter for dropped frames")
 delayed update of conn->seq_to_tap until the moment the corresponding
 frame has been successfully pushed out. This has the advantage that we
 immediately can make a new attempt to transmit a frame after a failed
 trasnmit, rather than waiting for the peer to later discover a gap and
 trigger the fast retransmit mechanism to solve the problem.
 
 This approach has turned out to cause a problem with spurious sequence
 number updates during peer-initiated retransmits, and we have realized
 it may not be the best way to solve the above issue.
 
 We now restore the previous method, by updating the said field at the
 moment a frame is added to the outqueue. To retain the advantage of
 having a quick re-attempt based on local failure detection, we now scan
 through the part of the outqueue that had do be dropped, and restore the
 sequence counter for each affected connection to the most appropriate
 value.
 
 Signed-off-by: Jon Maloy <jmaloy(a)redhat.com>
 
 ---
 v2: - Re-spun loop in tcp_revert_seq() and some other changes based on
       feedback from Stefano Brivio.
     - Added paranoid test to avoid that seq_to_tap becomes lower than
       seq_ack_from_tap.
 
 v3: - Identical to v2. Called v3 because it was embedded in a series
       with that version.
 
 v4: - In tcp_revert_seq(), we read the sequence number from the TCP
       header instead of keeping a copy in struct tcp_buf_seq_update.
     - Since the only remaining field in struct tcp_buf_seq_update is
       a pointer to struct tcp_tap_conn, we eliminate the struct
       altogether, and make the tcp6/tcp3_buf_seq_update arrays into
       arrays of said pointer.
     - Removed 'paranoid' test in tcp_revert_seq. If it happens, it
       is not fatal, and will be caught by other code anyway.
     - Separated from the series again.
 
 v5: - A couple of style issues.
 ---
  tcp.c | 61 ++++++++++++++++++++++++++++++++++++++---------------------
  1 file changed, 39 insertions(+), 22 deletions(-)
 
 diff --git a/tcp.c b/tcp.c
 index 21d0af0..3a2350a 100644
 --- a/tcp.c
 +++ b/tcp.c
 @@ -410,16 +410,6 @@ static int tcp_sock_ns		[NUM_PORTS][IP_VERSIONS];
   */
  static union inany_addr low_rtt_dst[LOW_RTT_TABLE_SIZE];
  
 -/**
 - * tcp_buf_seq_update - Sequences to update with length of frames once sent
 - * @seq:	Pointer to sequence number sent to tap-side, to be updated
 - * @len:	TCP payload length
 - */
 -struct tcp_buf_seq_update {
 -	uint32_t *seq;
 -	uint16_t len;
 -};
 -
  /* Static buffers */
  /**
   * struct tcp_payload_t - TCP header and data to send segments with payload
 @@ -461,7 +451,8 @@ static struct tcp_payload_t	tcp4_payload[TCP_FRAMES_MEM];
  
  static_assert(MSS4 <= sizeof(tcp4_payload[0].data), "MSS4 is greater than
65516");
  
 -static struct tcp_buf_seq_update tcp4_seq_update[TCP_FRAMES_MEM];
 +/* References tracking the owner connection of frames in the tap outqueue */
 +static struct tcp_tap_conn *tcp4_frame_conns[TCP_FRAMES_MEM];
  static unsigned int tcp4_payload_used;
  
  static struct tap_hdr		tcp4_flags_tap_hdr[TCP_FRAMES_MEM];
 @@ -483,7 +474,8 @@ static struct tcp_payload_t	tcp6_payload[TCP_FRAMES_MEM];
  
  static_assert(MSS6 <= sizeof(tcp6_payload[0].data), "MSS6 is greater than
65516");
  
 -static struct tcp_buf_seq_update tcp6_seq_update[TCP_FRAMES_MEM];
 +/* References tracking the owner connection of frames in the tap outqueue */
 +static struct tcp_tap_conn *tcp6_frame_conns[TCP_FRAMES_MEM];
  static unsigned int tcp6_payload_used;
  
  static struct tap_hdr		tcp6_flags_tap_hdr[TCP_FRAMES_MEM];
 @@ -1261,25 +1253,51 @@ static void tcp_flags_flush(const struct ctx *c)
  	tcp4_flags_used = 0;
  }
  
 +/**
 + * tcp_revert_seq() - Revert affected conn->seq_to_tap after failed transmission
 + * @conns:       Array of connection pointers corresponding to queued frames
 + * @frames:      Two-dimensional array containing queued frames with sub-iovs
 + * @num_frames:  Number of entries in the two arrays to be compared
 + */
 +static void tcp_revert_seq(struct tcp_tap_conn **conns, struct iovec
(*frames)[TCP_NUM_IOVS],
 +			   int num_frames)
 +{
 +	int i;
 +
 +	for (i = 0; i < num_frames; i++) {
 +		struct tcp_tap_conn *conn = conns[i];
 +		struct tcphdr *th = frames[i][TCP_IOV_PAYLOAD].iov_base;
 +		uint32_t seq = ntohl(th->seq);
 +
 +		if (SEQ_LE(conn->seq_to_tap, seq))
 +			continue;
 +
 +		conn->seq_to_tap = seq; 
Not worth a respin, but given the other simplifications to this, it
would be clearer to have:
	if (!SEQ_LE(conn->seq_to_tap, seq))
		conn->seq_to_tao = seq;

Rather than using continue;


...
  +	}
 +}
 +
  /**
   * tcp_payload_flush() - Send out buffers for segments with data
   * @c:		Execution context
   */
  static void tcp_payload_flush(const struct ctx *c)
  {
 -	unsigned i;
  	size_t m;
  
  	m = tap_send_frames(c, &tcp6_l2_iov[0][0], TCP_NUM_IOVS,
  			    tcp6_payload_used);
 -	for (i = 0; i < m; i++)
 -		*tcp6_seq_update[i].seq += tcp6_seq_update[i].len;
 +	if (m != tcp6_payload_used) {
 +		tcp_revert_seq(tcp6_frame_conns, &tcp6_l2_iov[m],
 +			       tcp6_payload_used - m); 
Hrm.. AFAICT tcp_revert_seq() is using the same indices into conns[]
and frames[].  But here, aren't you passing the frames array from
entry m onwards, but the conns array from 0 onwards?  Meaning that
revert_seq() might use the wrong connections for each frame.  I think
you either need
	tcp_revert_seq(&tcp6_frame_conns[m], &tcp6_l2_iov[m], ...)
Or else pass the unindexed arrays here, and take the start index as a
new parameter to tcp_revert_seq().

...
  +	}
  	tcp6_payload_used = 0;
  
  	m = tap_send_frames(c, &tcp4_l2_iov[0][0], TCP_NUM_IOVS,
  			    tcp4_payload_used);
 -	for (i = 0; i < m; i++)
 -		*tcp4_seq_update[i].seq += tcp4_seq_update[i].len;
 +	if (m != tcp4_payload_used) {
 +		tcp_revert_seq(tcp4_frame_conns, &tcp4_l2_iov[m],
 +			       tcp4_payload_used - m); 
Same thing here, of course.

...
  +	}
  	tcp4_payload_used = 0;
  }
  
 @@ -2129,10 +2147,11 @@ static int tcp_sock_consume(const struct tcp_tap_conn *conn,
uint32_t ack_seq)
  static void tcp_data_to_tap(const struct ctx *c, struct tcp_tap_conn *conn,
  			    ssize_t dlen, int no_csum, uint32_t seq)
  {
 -	uint32_t *seq_update = &conn->seq_to_tap;
  	struct iovec *iov;
  	size_t l4len;
  
 +	conn->seq_to_tap = seq + dlen; 
Now that we update seq_to_tap here, we don't really need seq as a
parameter any more, which would also simplify logic in the caller slightly.

...
   	if (CONN_V4(conn)) {
  		struct iovec *iov_prev = tcp4_l2_iov[tcp4_payload_used - 1];
  		const uint16_t *check = NULL;
 @@ -2142,8 +2161,7 @@ static void tcp_data_to_tap(const struct ctx *c, struct
tcp_tap_conn *conn,
  			check = &iph->check;
  		}
  
 -		tcp4_seq_update[tcp4_payload_used].seq = seq_update;
 -		tcp4_seq_update[tcp4_payload_used].len = dlen;
 +		tcp4_frame_conns[tcp4_payload_used] = conn;
  
  		iov = tcp4_l2_iov[tcp4_payload_used++];
  		l4len = tcp_l2_buf_fill_headers(c, conn, iov, dlen, check, seq);
 @@ -2151,8 +2169,7 @@ static void tcp_data_to_tap(const struct ctx *c, struct
tcp_tap_conn *conn,
  		if (tcp4_payload_used > TCP_FRAMES_MEM - 1)
  			tcp_payload_flush(c);
  	} else if (CONN_V6(conn)) {
 -		tcp6_seq_update[tcp6_payload_used].seq = seq_update;
 -		tcp6_seq_update[tcp6_payload_used].len = dlen;
 +		tcp6_frame_conns[tcp6_payload_used] = conn;
  
  		iov = tcp6_l2_iov[tcp6_payload_used++];
  		l4len = tcp_l2_buf_fill_headers(c, conn, iov, dlen, NULL, seq); 
-- 
David Gibson			| 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

On Fri, May 17, 2024 at 11:24:13AM -0400, Jon Maloy wrote:
...
 
 From
linux-6.9.0 the kernel will contain 
 commit 05ea491641d3 ("tcp: add support for
SO_PEEK_OFF socket option").
 
 This new feature makes is possible to call recv_msg(MSG_PEEK) and make
 it start reading data from a given offset set by the SO_PEEK_OFF socket
 option. This way, we can avoid repeated reading of already read bytes of
 a received message, hence saving read cycles when forwarding TCP
 messages in the host->name space direction.
 
 In this commit, we add functionality to leverage this feature when
 available, while we fall back to the previous behavior when not.
 
 Measurements with iperf3 shows that throughput increases with 15-20
 percent in the host->namespace direction when this feature is used.
 
 Signed-off-by: Jon Maloy <jmaloy(a)redhat.com> 
Reviewed-by: David Gibson <david(a)gibson.dropbear.id.au>

...
  
 ---
 v2: - Some smaller changes as suggested by David Gibson and Stefano Brivio.
     - Moved initial set_peek_offset(0) to only the locations where the socket is set
       to ESTABLISHED.
     - Removed the per-packet synchronization between sk_peek_off and
       already_sent. Instead only doing it in retransmit situations.
     - The problem I found when trouble shooting the occasionally occurring
       out of synch values between 'already_sent' and 'sk_peek_offset'
may
       have deeper implications that we may need to be investigate.
 
 v3: - Rebased to most recent version of tcp.c, plus the previous
       patch in this series.
     - Some changes based on feedback from PASST team
 
 v4: - Some small changes based on feedback from Stefan/David.
 
 v5: - Re-added accidentally dropped set_peek_offset() line.
       Thank you, David.
 ---
  tcp.c | 59 +++++++++++++++++++++++++++++++++++++++++++++++++++--------
  1 file changed, 51 insertions(+), 8 deletions(-)
 
 diff --git a/tcp.c b/tcp.c
 index 3a2350a..fa13292 100644
 --- a/tcp.c
 +++ b/tcp.c
 @@ -511,6 +511,9 @@ static struct iovec	tcp6_l2_iov		[TCP_FRAMES_MEM][TCP_NUM_IOVS];
  static struct iovec	tcp4_l2_flags_iov	[TCP_FRAMES_MEM][TCP_NUM_IOVS];
  static struct iovec	tcp6_l2_flags_iov	[TCP_FRAMES_MEM][TCP_NUM_IOVS];
  
 +/* Does the kernel support TCP_PEEK_OFF? */
 +static bool peek_offset_cap;
 +
  /* sendmsg() to socket */
  static struct iovec	tcp_iov			[UIO_MAXIOV];
  
 @@ -526,6 +529,20 @@ static_assert(ARRAY_SIZE(tc_hash) >= FLOW_MAX,
  int init_sock_pool4		[TCP_SOCK_POOL_SIZE];
  int init_sock_pool6		[TCP_SOCK_POOL_SIZE];
  
 +/**
 + * tcp_set_peek_offset() - Set SO_PEEK_OFF offset on a socket if supported
 + * @s:          Socket to update
 + * @offset:     Offset in bytes
 + */
 +static void tcp_set_peek_offset(int s, int offset)
 +{
 +	if (!peek_offset_cap)
 +		return;
 +
 +	if (setsockopt(s, SOL_SOCKET, SO_PEEK_OFF, &offset, sizeof(offset)))
 +		err("Failed to set SO_PEEK_OFF to %i in socket %i", offset, s);
 +}
 +
  /**
   * tcp_conn_epoll_events() - epoll events mask for given connection state
   * @events:	Current connection events
 @@ -1273,6 +1290,7 @@ static void tcp_revert_seq(struct tcp_tap_conn **conns, struct
iovec (*frames)[T
  			continue;
  
  		conn->seq_to_tap = seq;
 +		tcp_set_peek_offset(conn->sock, seq - conn->seq_ack_from_tap);
  	}
  }
  
 @@ -2199,14 +2217,15 @@ static int tcp_data_from_sock(struct ctx *c, struct tcp_tap_conn
*conn)
  	uint32_t already_sent, seq;
  	struct iovec *iov;
  
 +	/* How much have we read/sent since last received ack ? */
  	already_sent = conn->seq_to_tap - conn->seq_ack_from_tap;
 -
  	if (SEQ_LT(already_sent, 0)) {
  		/* RFC 761, section 2.1. */
  		flow_trace(conn, "ACK sequence gap: ACK for %u, sent: %u",
  			   conn->seq_ack_from_tap, conn->seq_to_tap);
  		conn->seq_to_tap = conn->seq_ack_from_tap;
  		already_sent = 0;
 +		tcp_set_peek_offset(s, 0);
  	}
  
  	if (!wnd_scaled || already_sent >= wnd_scaled) {
 @@ -2224,11 +2243,16 @@ static int tcp_data_from_sock(struct ctx *c, struct tcp_tap_conn
*conn)
  		iov_rem = (wnd_scaled - already_sent) % mss;
  	}
  
 -	mh_sock.msg_iov = iov_sock;
 -	mh_sock.msg_iovlen = fill_bufs + 1;
 -
 -	iov_sock[0].iov_base = tcp_buf_discard;
 -	iov_sock[0].iov_len = already_sent;
 +	/* Prepare iov according to kernel capability */
 +	if (!peek_offset_cap) {
 +		mh_sock.msg_iov = iov_sock;
 +		iov_sock[0].iov_base = tcp_buf_discard;
 +		iov_sock[0].iov_len = already_sent;
 +		mh_sock.msg_iovlen = fill_bufs + 1;
 +	} else {
 +		mh_sock.msg_iov = &iov_sock[1];
 +		mh_sock.msg_iovlen = fill_bufs;
 +	}
  
  	if (( v4 && tcp4_payload_used + fill_bufs > TCP_FRAMES_MEM) ||
  	    (!v4 && tcp6_payload_used + fill_bufs > TCP_FRAMES_MEM)) {
 @@ -2269,7 +2293,10 @@ static int tcp_data_from_sock(struct ctx *c, struct tcp_tap_conn
*conn)
  		return 0;
  	}
  
 -	sendlen = len - already_sent;
 +	sendlen = len;
 +	if (!peek_offset_cap)
 +		sendlen -= already_sent;
 +
  	if (sendlen <= 0) {
  		conn_flag(c, conn, STALLED);
  		return 0;
 @@ -2440,6 +2467,7 @@ static int tcp_data_from_tap(struct ctx *c, struct tcp_tap_conn
*conn,
  			   "fast re-transmit, ACK: %u, previous sequence: %u",
  			   max_ack_seq, conn->seq_to_tap);
  		conn->seq_to_tap = max_ack_seq;
 +		tcp_set_peek_offset(conn->sock, 0);
  		tcp_data_from_sock(c, conn);
  	}
  
 @@ -2532,6 +2560,7 @@ static void tcp_conn_from_sock_finish(struct ctx *c, struct
tcp_tap_conn *conn,
  	conn->seq_ack_to_tap = conn->seq_from_tap;
  
  	conn_event(c, conn, ESTABLISHED);
 +	tcp_set_peek_offset(conn->sock, 0);
  
  	/* The client might have sent data already, which we didn't
  	 * dequeue waiting for SYN,ACK from tap -- check now.
 @@ -2612,6 +2641,7 @@ int tcp_tap_handler(struct ctx *c, uint8_t pif, sa_family_t af,
  			goto reset;
  
  		conn_event(c, conn, ESTABLISHED);
 +		tcp_set_peek_offset(conn->sock, 0);
  
  		if (th->fin) {
  			conn->seq_from_tap++;
 @@ -2865,6 +2895,7 @@ void tcp_timer_handler(struct ctx *c, union epoll_ref ref)
  			flow_dbg(conn, "ACK timeout, retry");
  			conn->retrans++;
  			conn->seq_to_tap = conn->seq_ack_from_tap;
 +			tcp_set_peek_offset(conn->sock, 0);
  			tcp_data_from_sock(c, conn);
  			tcp_timer_ctl(c, conn);
  		}
 @@ -3156,7 +3187,8 @@ static void tcp_sock_refill_init(const struct ctx *c)
   */
  int tcp_init(struct ctx *c)
  {
 -	unsigned b;
 +	unsigned int b, optv = 0;
 +	int s;
  
  	for (b = 0; b < TCP_HASH_TABLE_SIZE; b++)
  		tc_hash[b] = FLOW_SIDX_NONE;
 @@ -3180,6 +3212,17 @@ int tcp_init(struct ctx *c)
  		NS_CALL(tcp_ns_socks_init, c);
  	}
  
 +	/* Probe for SO_PEEK_OFF support */
 +	s = socket(AF_INET, SOCK_STREAM | SOCK_CLOEXEC, IPPROTO_TCP);
 +	if (s < 0) {
 +		warn("Temporary TCP socket creation failed");
 +	} else {
 +		if (!setsockopt(s, SOL_SOCKET, SO_PEEK_OFF, &optv, sizeof(int)))
 +			peek_offset_cap = true;
 +		close(s);
 +	}
 +	info("SO_PEEK_OFF%ssupported", peek_offset_cap ? " " : " not
");
 +
  	return 0;
  }
   
-- 
David Gibson			| 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

On Fri, May 17, 2024 at 11:24:14AM -0400, Jon Maloy wrote:
...
  A bug in kernel TCP may lead to a deadlock where a
zero window is sent
 from the peer, while it is unable to send out window updates even after
 reads have freed up enough buffer space to permit a larger window.
 In this situation, new window advertisemnts from the peer can only be
 triggered by packets arriving from this side.
 
 However, such packets are never sent, because the zero-window condition
 currently prevents this side from sending out any packets whatsoever
 to the peer.
 
 We notice that the above bug is triggered *only* after the peer has
 dropped an arriving packet because of severe memory squeeze, and that we
 hence always enter a retransmission situation when this occurs. This
 also means that it goes against the RFC 9293 recommendation that a
 previously advertised window never should shrink.
 
 RFC 9293 gives the solution to this situation. In chapter 3.6.1 we find
 the following statement:
 "A TCP receiver SHOULD NOT shrink the window, i.e., move the right
 window edge to the left (SHLD-14). However, a sending TCP peer MUST
 be robust against window shrinking, which may cause the
 "usable window" (see Section 3.8.6.2.1) to become negative (MUST-34).
 
 If this happens, the sender SHOULD NOT send new data (SHLD-15), but
 SHOULD retransmit normally the old unacknowledged data between SND.UNA
 and SND.UNA+SND.WND (SHLD-16). The sender MAY also retransmit old data
 beyond SND.UNA+SND.WND (MAY-7)" 
So... I'm beginning to think this section of the rfc isn't really
helpful or useful here.  For starters, it doesn't seem to cover all of
what we're trying to do here - particularly the fact that we try to
send keepalive probes when in this situation...

...
  We never see the window become negative, but we
interpret this as a
 recommendation to use the previously available window during
 retransmission even when the currently advertised window is zero. 
... but also, looking at the RFC, I'm really not convinced of this
interpretation.  SND.WND generally refers to the last window we've
seen advertised by the guest, and I don't see any indication that in
this specific case we should instead consider the previous version it
had.

Indeed the "usable window" value it's discussing is elsewhere
described in terms of SND.WND, and if we used the previous SND.WND
value it would *not* become negative.

I believe that last MAY-7 bit means we're not violating the RFC by
using the previous window edge, but I don't think there's anything
there to suggest we must or should be doing so.

[In fact, I wonder if the reason behind MAY-7 is that it allows an
 implementation to satisfy this by just ignoring ignore window updates
 which would move the right edge backwards]

So.. moving on from the RFC to what we actually need to do to
workaround this bug.  Do we actually need anything more than
continuing to send keep-alive probes even when the window is zero?

...
  We use the above mechanism only for timer-induced
retransmits, while
 the fast-retransmit mechanism won't trigger on this condition.
 
 It should be noted that although this solves the problem we have at
 hand, it is not a genuine solution to the kernel bug. There may well
 be TCP stacks around in other OS-es which don't do this, nor have
 keep-alive probing as an alternatve way to solve the situation.
 
 Signed-off-by: Jon Maloy <jmaloy(a)redhat.com>
 
 ---
 v2: - Using previously advertised window during retransmission, instead
       highest send sequencece number in the cycle.
 v3: - Rebased to newest code
     - Changes based on feedback from PASST team
     - Sending out empty probe message at timer expiration when
       we are not in retransmit situation.
 v4: - Some small changes based on feedback from PASST team.
     - Replaced fast retransmit with a one-time 'fast probe' when
       window is zero.
 v5: - Gave up on 'fast probing' for now. When I got the sequence
       numbers right in the flag message (after having emptied the tap
       queue), it turns out an empty message does *not* force a new peer
       window update as was my previous understanding of the code.
     - Added cppcheck suppression line (which I was unable to verify)
       as suggested by S. Brivio.
     - Removed sending of empty probe when window from tap side is zero.
       It looks pointless at the moment, at least for solving the above
       described situation.
 v6: - Ensure that arrival of new data doesn´t cause us to ignore a
       zero-window situation.
     - Removed the pointless probing referred to in v5 comment.
 ---
  tcp.c      | 26 ++++++++++++++++++++------
  tcp_conn.h |  2 ++
  2 files changed, 22 insertions(+), 6 deletions(-)
 
 diff --git a/tcp.c b/tcp.c
 index fa13292..38c3480 100644
 --- a/tcp.c
 +++ b/tcp.c
 @@ -1764,9 +1764,17 @@ static void tcp_get_tap_ws(struct tcp_tap_conn *conn,
   */
  static void tcp_tap_window_update(struct tcp_tap_conn *conn, unsigned wnd)
  {
 +	uint32_t wnd_edge;
 +
  	wnd = MIN(MAX_WINDOW, wnd << conn->ws_from_tap);
 +	/* cppcheck-suppress [knownConditionTrueFalse, unmatchedSuppression] */ 
If I recall from earlier, we thought this suppression was needed
because of the cppcheck bug referenced in tcp_update_seqack_wnd().  If
that's the case we need something like that comment here as well:
knownConditionTrueFalse is not a check we should be suppressing
lightly.

But also... is it actually that bug?  In that case the check tripped
when we did an if based on the result of the MIN - it thought it was
always zero.  But here the suppression is on the MIN itself, which
suggests something different.  Is it instead that cppcheck is managing
to deduce that wnd >> conn->ws_from_tap cannot be greater than
USHRT_MAX.  Which should indeed be the case, although I can't quickly
see how you'd statically deduce it.

I'm also not sure why this is showing up now, because these lines
aren't changed.

...
  + 
I also don't think inserting a blank line between the suppression and
the line where the error is occuring is a good idea.

>  	conn->wnd_from_tap = MIN(wnd >> conn->ws_from_tap, USHRT_MAX);
>  
> +	wnd_edge = conn->seq_ack_from_tap + wnd;
> +	if (wnd && SEQ_GT(wnd_edge, conn->seq_wnd_edge_from_tap))
> +		conn->seq_wnd_edge_from_tap = wnd_edge;
...
  + 
>  	/* FIXME: reflect the tap-side
receiver's window back to the sock-side
>  	 * sender by adjusting SO_RCVBUF? */
>  }
> @@ -1799,6 +1807,7 @@ static void tcp_seq_init(const struct ctx *c, struct
tcp_tap_conn *conn,
>  	ns = (now->tv_sec * 1000000000 + now->tv_nsec) >> 5;
>  
>  	conn->seq_to_tap = ((uint32_t)(hash >> 32) ^ (uint32_t)hash) + ns;
> +	conn->seq_wnd_edge_from_tap = conn->seq_to_tap;
>  }
>  
>  /**
> @@ -2208,13 +2217,12 @@ static void tcp_data_to_tap(const struct ctx *c, struct
tcp_tap_conn *conn,
>   */
>  static int tcp_data_from_sock(struct ctx *c, struct tcp_tap_conn *conn)
>  {
> -	uint32_t wnd_scaled = conn->wnd_from_tap << conn->ws_from_tap;
>  	int fill_bufs, send_bufs = 0, last_len, iov_rem = 0;
>  	int sendlen, len, dlen, v4 = CONN_V4(conn);
> +	uint32_t already_sent, max_send, seq;
>  	int s = conn->sock, i, ret = 0;
>  	struct msghdr mh_sock = { 0 };
>  	uint16_t mss = MSS_GET(conn);
> -	uint32_t already_sent, seq;
>  	struct iovec *iov;
>  
>  	/* How much have we read/sent since last received ack ? */
> @@ -2228,19 +2236,24 @@ static int tcp_data_from_sock(struct ctx *c, struct
tcp_tap_conn *conn)
>  		tcp_set_peek_offset(s, 0);
>  	}
>  
> -	if (!wnd_scaled || already_sent >= wnd_scaled) {
> +	/* How much are we still allowed to send within current window ? */
> +	max_send = conn->seq_wnd_edge_from_tap - conn->seq_to_tap;
> +	if (SEQ_LE(max_send, 0)) {

Although the maths probably works out correctly, I dislike using
SEQ_LE on sequence differences here, rather that using SEQ_LE directly
on seq_wnd_edge_from_tap and seq_to_tap.

...
  +		flow_trace(conn, "Window full: right edge: %u,
sent: %u",
 +			   conn->seq_wnd_edge_from_tap, conn->seq_to_tap);
 +		conn->seq_wnd_edge_from_tap = conn->seq_to_tap; 
So, here we pull seq_wnd_edge_from_tap back in line with seq_to_tap.
Which might be before even the "current" window of seq_ack_to_tap +
wnd_scaled.  Which means there's a pretty brief window in which
seq_wnd_edge_from_tap will actually be beyond the latest window.  It's
not clear to me why that brief window is important - or why getting
more data from the socket side would be relevant to finishing that
window.

>  		conn_flag(c, conn, STALLED);
>  		conn_flag(c, conn, ACK_FROM_TAP_DUE);
>  		return 0;
>  	}
>  
>  	/* Set up buffer descriptors we'll fill completely and partially. */
> -	fill_bufs = DIV_ROUND_UP(wnd_scaled - already_sent, mss);
> +	fill_bufs = DIV_ROUND_UP(max_send,  mss);
>  	if (fill_bufs > TCP_FRAMES) {
>  		fill_bufs = TCP_FRAMES;
>  		iov_rem = 0;
>  	} else {
> -		iov_rem = (wnd_scaled - already_sent) % mss;
> +		iov_rem = max_send % mss;
>  	}
>  
>  	/* Prepare iov according to kernel capability */
> @@ -2347,6 +2360,7 @@ err:
>   *
>   * Return: count of consumed packets
>   */
...
  + 
Spurious whitespace change.

...
   static int tcp_data_from_tap(struct ctx *c, struct
tcp_tap_conn *conn,
  			      const struct pool *p, int idx)
  {
 @@ -2950,7 +2964,7 @@ void tcp_sock_handler(struct ctx *c, union epoll_ref ref, uint32_t
events)
  		if (events & (EPOLLRDHUP | EPOLLHUP))
  			conn_event(c, conn, SOCK_FIN_RCVD);
  
 -		if (events & EPOLLIN)
 +		if (events & EPOLLIN && conn->wnd_from_tap) 
Hrm.  If we don't even enter tcp_data_from_sock() when there's no
window, doesn't that mean we won't hit the handling for the max_send <
0 case, we won't set STALLED, won't switch the epoll flags for the
socket to edge triggered mode and will therefore just busy loop on
EPOLLIN socket events until the window re-opens.

...
   			tcp_data_from_sock(c, conn);
  
  		if (events & EPOLLOUT)
 diff --git a/tcp_conn.h b/tcp_conn.h
 index d280b22..5cbad2a 100644
 --- a/tcp_conn.h
 +++ b/tcp_conn.h
 @@ -30,6 +30,7 @@
   * @wnd_to_tap:		Sending window advertised to tap, unscaled (as sent)
   * @seq_to_tap:		Next sequence for packets to tap
   * @seq_ack_from_tap:	Last ACK number received from tap
 + * @seq_wnd_edge_from_tap: Right edge of last non-zero window from tap
   * @seq_from_tap:	Next sequence for packets from tap (not actually sent)
   * @seq_ack_to_tap:	Last ACK number sent to tap
   * @seq_init_from_tap:	Initial sequence number from tap
 @@ -101,6 +102,7 @@ struct tcp_tap_conn {
  
  	uint32_t	seq_to_tap;
  	uint32_t	seq_ack_from_tap;
 +	uint32_t	seq_wnd_edge_from_tap;
  	uint32_t	seq_from_tap;
  	uint32_t	seq_ack_to_tap;
  	uint32_t	seq_init_from_tap; 
-- 
David Gibson			| 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