Re: [PATCH v5 1/8] tcp: extract buffer management from tcp_send_flag()

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Jun 12, 2024 at 12:09:04AM +0200, Stefano Brivio wrote:
> On Wed,  5 Jun 2024 17:21:22 +0200
> Laurent Vivier <lvivier@redhat.com> wrote:
> 
> > This commit isolates the internal data structure management used for storing
> > data (e.g., tcp4_l2_flags_iov[], tcp6_l2_flags_iov[], tcp4_flags_ip[],
> > tcp4_flags[], ...) from the tcp_send_flag() function. The extracted
> > functionality is relocated to a new function named tcp_fill_flag_header().
> > 
> > tcp_fill_flag_header() is now a generic function that accepts parameters such
> > as struct tcphdr and a data pointer. tcp_send_flag() utilizes this parameter to
> > pass memory pointers from tcp4_l2_flags_iov[] and tcp6_l2_flags_iov[].
> > 
> > This separation sets the stage for utilizing tcp_fill_flag_header() to
> > set the memory provided by the guest via vhost-user in future developments.
> > 
> > Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> > ---
> >  tcp.c | 63 ++++++++++++++++++++++++++++++++++++-----------------------
> >  1 file changed, 39 insertions(+), 24 deletions(-)
> > 
> > diff --git a/tcp.c b/tcp.c
> > index 06acb41e4d90..68d4afa05a36 100644
> > --- a/tcp.c
> > +++ b/tcp.c
> > @@ -1549,24 +1549,25 @@ static void tcp_update_seqack_from_tap(const struct ctx *c,
> >  }
> >  
> >  /**
> > - * tcp_send_flag() - Send segment with flags to tap (no payload)
> > + * tcp_fill_flag_header() - Prepare header for flags-only segment (no payload)
> >   * @c:		Execution context
> >   * @conn:	Connection pointer
> >   * @flags:	TCP flags: if not set, send segment only if ACK is due
> > + * @th:		TCP header to update
> > + * @data:	buffer to store TCP option
> > + * @optlen:	size of the TCP option buffer
> 
> Now, this becomes an output parameter if SYN is set in flags, but it's
> otherwise an input parameter (and it must be zero, otherwise the data
> offset field we send will be wrong).
> 
> I think having it always as output parameter (that is, setting it to
> zero on non-SYN in this function, not in the caller) would be less
> fragile and easier to describe in the comment, too.

I agree.

> Or, even simpler, pass it as input parameter, and calculate it in the
> caller. The caller sets 'flags' anyway.

Eh... it seems to me that the knowledge of how to translate the flags
bits to a specific length better belongs here, so I don't know that's
a great idea.

> >   *
> > - * Return: negative error code on connection reset, 0 otherwise
> > + * Return: < 0 error code on connection reset,
> > + *           0 if there is no flag to send
> 
> As you use one tab to indent "1 otherwise" below, you could use one
> here as well.
> 
> > + *	     1 otherwise
> >   */
> > -static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
> > +static int tcp_fill_flag_header(struct ctx *c, struct tcp_tap_conn *conn,
> > +				int flags, struct tcphdr *th, char *data,
> > +				size_t *optlen)
> >  {
> > -	struct tcp_flags_t *payload;
> >  	struct tcp_info tinfo = { 0 };
> >  	socklen_t sl = sizeof(tinfo);
> >  	int s = conn->sock;
> > -	size_t optlen = 0;
> > -	struct tcphdr *th;
> > -	struct iovec *iov;
> > -	size_t l4len;
> > -	char *data;
> >  
> >  	if (SEQ_GE(conn->seq_ack_to_tap, conn->seq_from_tap) &&
> >  	    !flags && conn->wnd_to_tap)
> > @@ -1588,20 +1589,11 @@ static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
> >  	if (!tcp_update_seqack_wnd(c, conn, flags, &tinfo) && !flags)
> >  		return 0;
> >  
> > -	if (CONN_V4(conn))
> > -		iov = tcp4_l2_flags_iov[tcp4_flags_used++];
> > -	else
> > -		iov = tcp6_l2_flags_iov[tcp6_flags_used++];
> > -
> > -	payload = iov[TCP_IOV_PAYLOAD].iov_base;
> > -	th = &payload->th;
> > -	data = payload->opts;
> > -
> >  	if (flags & SYN) {
> >  		int mss;
> >  
> >  		/* Options: MSS, NOP and window scale (8 bytes) */
> > -		optlen = OPT_MSS_LEN + 1 + OPT_WS_LEN;
> > +		*optlen = OPT_MSS_LEN + 1 + OPT_WS_LEN;
> >  
> >  		*data++ = OPT_MSS;
> >  		*data++ = OPT_MSS_LEN;
> > @@ -1635,17 +1627,13 @@ static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
> >  		flags |= ACK;
> >  	}
> >  
> > -	th->doff = (sizeof(*th) + optlen) / 4;
> > +	th->doff = (sizeof(*th) + *optlen) / 4;
> >  
> >  	th->ack = !!(flags & ACK);
> >  	th->rst = !!(flags & RST);
> >  	th->syn = !!(flags & SYN);
> >  	th->fin = !!(flags & FIN);
> >  
> > -	l4len = tcp_l2_buf_fill_headers(c, conn, iov, optlen, NULL,
> > -					conn->seq_to_tap);
> > -	iov[TCP_IOV_PAYLOAD].iov_len = l4len;
> > -
> >  	if (th->ack) {
> >  		if (SEQ_GE(conn->seq_ack_to_tap, conn->seq_from_tap))
> >  			conn_flag(c, conn, ~ACK_TO_TAP_DUE);
> > @@ -1660,6 +1648,33 @@ static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
> >  	if (th->fin || th->syn)
> >  		conn->seq_to_tap++;
> >  
> > +	return 1;
> > +}
> > +
> > +static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
> > +{
> > +	struct tcp_flags_t *payload;
> > +	size_t optlen = 0;
> > +	struct iovec *iov;
> > +	size_t l4len;
> > +	int ret;
> > +
> > +	if (CONN_V4(conn))
> > +		iov = tcp4_l2_flags_iov[tcp4_flags_used++];
> > +	else
> > +		iov = tcp6_l2_flags_iov[tcp6_flags_used++];
> > +
> > +	payload = iov[TCP_IOV_PAYLOAD].iov_base;
> > +
> > +	ret = tcp_fill_flag_header(c, conn, flags, &payload->th,
> > +				   payload->opts, &optlen);
> > +	if (ret <= 0)
> > +		return ret;
> > +
> > +	l4len = tcp_l2_buf_fill_headers(c, conn, iov, optlen, NULL,
> > +					conn->seq_to_tap);
> > +	iov[TCP_IOV_PAYLOAD].iov_len = l4len;
> > +
> >  	if (flags & DUP_ACK) {
> >  		struct iovec *dup_iov;
> >  		int i;
> 

-- 
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

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