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 05, 2024 at 05:21:22PM +0200, Laurent Vivier 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.

Thanks for the commit message, it makes this much clearer.

I have a number of comments below, but they're basically all cosmetic.

> 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)

I don't love the name tcp_fill_flag_header(), although it's not
terrible.  Maybe tcp_prepare_flags() would be better?

>   * @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

Worth noting this is an output parameter here...

>   *
> - * Return: negative error code on connection reset, 0 otherwise
> + * Return: < 0 error code on connection reset,
> + *           0 if there is no flag to send
> + *	     1 otherwise

.. or, since optlen will always be positive on success cases, you
could just return it.

>   */
> -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;
> +}
> +

Function comment, please.

> +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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