Re: [PATCH v7 1/8] tcp: extract buffer management from tcp_send_flag()

[Laurent Vivier <lvivier@redhat.com>]

On 13/06/2024 14:36, 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.

Forgot to update the comment: s/tcp_fill_flag_header/tcp_prepare_flags/

Could you update on merge?

Thanks,
Laurent

> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>   tcp.c | 78 +++++++++++++++++++++++++++++++++++++++++------------------
>   1 file changed, 54 insertions(+), 24 deletions(-)
> 
> diff --git a/tcp.c b/tcp.c
> index dd8d46e08628..e40f42259451 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -1567,24 +1567,25 @@ static void tcp_update_seqack_from_tap(const struct ctx *c,
>   }
>   
>   /**
> - * tcp_send_flag() - Send segment with flags to tap (no payload)
> + * tcp_prepare_flags() - 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 (output parameter)
>    *
> - * 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
>    */
> -static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
> +static int tcp_prepare_flags(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)
> @@ -1606,20 +1607,12 @@ 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;
> -
> +	*optlen = 0;
>   	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;
> @@ -1653,17 +1646,13 @@ 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);
> @@ -1678,6 +1667,47 @@ 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;
> +}
> +
> +/**
> + * tcp_send_flag() - Send segment with flags to tap (no payload)
> + * @c:         Execution context
> + * @conn:      Connection pointer
> + * @flags:     TCP flags: if not set, send segment only if ACK is due
> + *
> + * Return: negative error code on connection reset, 0 otherwise
> + */
> +static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
> +{
> +	struct tcp_flags_t *payload;
> +	struct iovec *iov;
> +	size_t optlen;
> +	size_t l4len;
> +	uint32_t seq;
> +	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;
> +
> +	seq = conn->seq_to_tap;
> +	ret = tcp_prepare_flags(c, conn, flags, &payload->th,
> +				payload->opts, &optlen);
> +	if (ret <= 0) {
> +		if (CONN_V4(conn))
> +			tcp4_flags_used--;
> +		else
> +			tcp6_flags_used--;
> +		return ret;
> +	}
> +
> +	l4len = tcp_l2_buf_fill_headers(c, conn, iov, optlen, NULL, seq);
> +	iov[TCP_IOV_PAYLOAD].iov_len = l4len;
> +
>   	if (flags & DUP_ACK) {
>   		struct iovec *dup_iov;
>   		int i;