Re: [RFC] tcp: Replace TCP buffer structure by an iovec array

[Stefano Brivio <sbrivio@redhat.com>]

On Mon, 11 Mar 2024 14:33:56 +0100
Laurent Vivier <lvivier@redhat.com> wrote:

> To be able to provide pointers to TCP headers and IP headers without
> worrying about alignment in the structure, split the structure into
> several arrays and point to each part of the frame using an iovec array.
> 
> Using iovec also allows us to simply ignore the first entry when the
> vnet length header is not needed. And as the payload buffer contains
> only the TCP header and the TCP data we can increase the size of the
> TCP data to USHRT_MAX - sizeof(struct tcphdr).
> 
> As a side effect, these changes improve performance by a factor of
> x1.5.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  tap.c | 104 ++++++++++++++
>  tap.h |  16 +++
>  tcp.c | 429 ++++++++++++++++++++++++----------------------------------
>  3 files changed, 296 insertions(+), 253 deletions(-)
> 
> diff --git a/tap.c b/tap.c
> index c7b9372668ec..afd01cffbed6 100644
> --- a/tap.c
> +++ b/tap.c
> @@ -350,6 +350,30 @@ static size_t tap_send_frames_pasta(const struct ctx *c,
>  	return i;
>  }
>  
> +/**
> + * tap_send_iov_pasta() - Send out multiple prepared frames
> + * @c:		Execution context
> + * @iov:	Array of frames, each frames is divided in an array of iovecs.
> + *              The first entry of the iovec is ignored

Fits on single lines (I think it's more readable):

 * @c:		Execution context
 * @iov:	Array of frames, each one a vector of parts, TCP_IOV_VNET unused
 * @n:		Number of frames in @iov

> + * @n:		Number of frames in @iov
> + *
> + * Return: number of frames actually sent
> + */
> +static size_t tap_send_iov_pasta(const struct ctx *c,
> +				 struct iovec iov[][TCP_IOV_NUM], size_t n)
> +{
> +	unsigned int i;
> +
> +	for (i = 0; i < n; i++) {
> +		if (!tap_send_frames_pasta(c, &iov[i][TCP_IOV_ETH],
> +					   TCP_IOV_NUM - TCP_IOV_ETH))
> +			break;
> +	}
> +
> +	return i;
> +
> +}
> +
>  /**
>   * tap_send_frames_passt() - Send multiple frames to the passt tap
>   * @c:		Execution context
> @@ -390,6 +414,42 @@ static size_t tap_send_frames_passt(const struct ctx *c,
>  	return i;
>  }
>  
> +/**
> + * tap_send_iov_passt() - Send out multiple prepared frames

...I would argue that this function prepares frames as well. Maybe:

 * tap_send_iov_passt() - Prepare TCP_IOV_VNET parts and send multiple frames

> + * @c:		Execution context
> + * @iov:	Array of frames, each frames is divided in an array of iovecs.
> + *              The first entry of the iovec is updated to point to an
> + *              uint32_t storing the frame length.

 * @iov:	Array of frames, each one a vector of parts, TCP_IOV_VNET blank

> + * @n:		Number of frames in @iov
> + *
> + * Return: number of frames actually sent
> + */
> +static size_t tap_send_iov_passt(const struct ctx *c,
> +				 struct iovec iov[][TCP_IOV_NUM],
> +				 size_t n)
> +{
> +	unsigned int i;
> +
> +	for (i = 0; i < n; i++) {
> +		uint32_t vnet_len;
> +		int j;
> +
> +		vnet_len = 0;

This could be initialised in the declaration (yes, it's "reset" at
every loop iteration).

> +		for (j = TCP_IOV_ETH; j < TCP_IOV_NUM; j++)
> +			vnet_len += iov[i][j].iov_len;
> +
> +		vnet_len = htonl(vnet_len);
> +		iov[i][TCP_IOV_VNET].iov_base = &vnet_len;
> +		iov[i][TCP_IOV_VNET].iov_len = sizeof(vnet_len);
> +
> +		if (!tap_send_frames_passt(c, iov[i], TCP_IOV_NUM))

...which would now send a single frame at a time, but actually it can
already send everything in one shot because it's using sendmsg(), if you
move it outside of the loop and do something like (untested):

	return tap_send_frames_passt(c, iov, TCP_IOV_NUM * n);

> +			break;
> +	}
> +
> +	return i;
> +
> +}
> +
>  /**
>   * tap_send_frames() - Send out multiple prepared frames
>   * @c:		Execution context
> @@ -418,6 +478,50 @@ size_t tap_send_frames(const struct ctx *c, const struct iovec *iov, size_t n)
>  	return m;
>  }
>  
> +/**
> + * tap_send_iov() - Send out multiple prepared frames
> + * @c:		Execution context
> + * @iov:	Array of frames, each frames is divided in an array of iovecs.
> + * 		iovec array is:

I think this description should be moved before the defines -- it could
even be an enum with fixed numbers, and I just realised that in
kernel-doc enums should be documented in the same way as structs (we
didn't do that so far), that is:

/**
 * enum tcp_iov_parts - I/O vector parts for one TCP frame
 * @TCP_IOV_NET:	vnet length: host order, 32-bit frame length descriptor
 * ...
 */
enum tcp_iov_parts {
	/* vnet length (host order, 32-bit length descriptor) */
	TCP_IOV_VNET		= 0,
	[...]
};

> + * 		TCP_IOV_VNET	(0)	vnet length
> + * 		TCP_IOV_ETH	(1)	ethernet header
> + * 		TCP_IOV_IP	(2)	IP (v4/v6) header
> + *		TCP_IOV_PAYLOAD	(3)	IP payload (TCP header + data)
> + *		TCP_IOV_NUM (4) is the number of entries in the iovec array
> + *		TCP_IOV_VNET entry is updated with passt, ignored with pasta.
> + * @n:		Number of frames in @iov
> + *
> + * Return: number of frames actually sent
> + */
> +size_t tap_send_iov(const struct ctx *c, struct iovec iov[][TCP_IOV_NUM],
> +		    size_t n)
> +{
> +	size_t m;
> +	unsigned int i;
> +
> +	if (!n)
> +		return 0;
> +
> +	switch (c->mode) {
> +	case MODE_PASST:
> +		m = tap_send_iov_passt(c, iov, n);
> +		break;
> +	case MODE_PASTA:
> +		m = tap_send_iov_pasta(c, iov, n);
> +		break;
> +	default:
> +		ASSERT(0);
> +	}
> +
> +	if (m < n)
> +		debug("tap: failed to send %zu frames of %zu", n - m, n);
> +
> +	for (i = 0; i < m; i++)
> +		pcap_iov(&iov[i][TCP_IOV_ETH], TCP_IOV_NUM - TCP_IOV_ETH);
> +
> +	return m;
> +}
> +
>  /**
>   * eth_update_mac() - Update tap L2 header with new Ethernet addresses
>   * @eh:		Ethernet headers to update
> diff --git a/tap.h b/tap.h
> index 437b9aa2b43f..2d8e7aa1101d 100644
> --- a/tap.h
> +++ b/tap.h
> @@ -6,6 +6,20 @@
>  #ifndef TAP_H
>  #define TAP_H
>  
> +/*
> + * TCP frame iovec array:
> + * TCP_IOV_VNET		vnet length
> + * TCP_IOV_ETH		ethernet header
> + * TCP_IOV_IP		IP (v4/v6) header
> + * TCP_IOV_PAYLOAD	IP payload (TCP header + data)
> + * TCP_IOV_NUM is the number of entries in the iovec array
> + */
> +#define TCP_IOV_VNET	0
> +#define TCP_IOV_ETH	1
> +#define TCP_IOV_IP	2
> +#define TCP_IOV_PAYLOAD	3
> +#define TCP_IOV_NUM	4
> +
>  /**
>   * struct tap_hdr - L2 and tap specific headers
>   * @vnet_len:	Frame length (for qemu socket transport)
> @@ -74,6 +88,8 @@ void tap_icmp6_send(const struct ctx *c,
>  		    const void *in, size_t len);
>  int tap_send(const struct ctx *c, const void *data, size_t len);
>  size_t tap_send_frames(const struct ctx *c, const struct iovec *iov, size_t n);
> +size_t tap_send_iov(const struct ctx *c, struct iovec iov[][TCP_IOV_NUM],
> +		    size_t n);
>  void eth_update_mac(struct ethhdr *eh,
>  		    const unsigned char *eth_d, const unsigned char *eth_s);
>  void tap_listen_handler(struct ctx *c, uint32_t events);
> diff --git a/tcp.c b/tcp.c
> index d5eedf4d0138..5c8488108ef7 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -318,39 +318,7 @@
>  
>  /* MSS rounding: see SET_MSS() */
>  #define MSS_DEFAULT			536
> -
> -struct tcp4_l2_head {	/* For MSS4 macro: keep in sync with tcp4_l2_buf_t */
> -#ifdef __AVX2__
> -	uint8_t pad[26];
> -#else
> -	uint8_t pad[2];
> -#endif
> -	struct tap_hdr taph;
> -	struct iphdr iph;
> -	struct tcphdr th;
> -#ifdef __AVX2__
> -} __attribute__ ((packed, aligned(32)));
> -#else
> -} __attribute__ ((packed, aligned(__alignof__(unsigned int))));
> -#endif
> -
> -struct tcp6_l2_head {	/* For MSS6 macro: keep in sync with tcp6_l2_buf_t */
> -#ifdef __AVX2__
> -	uint8_t pad[14];
> -#else
> -	uint8_t pad[2];
> -#endif
> -	struct tap_hdr taph;
> -	struct ipv6hdr ip6h;
> -	struct tcphdr th;
> -#ifdef __AVX2__
> -} __attribute__ ((packed, aligned(32)));
> -#else
> -} __attribute__ ((packed, aligned(__alignof__(unsigned int))));
> -#endif

I'm so glad to see this going away.

> -#define MSS4	ROUND_DOWN(USHRT_MAX - sizeof(struct tcp4_l2_head), 4)
> -#define MSS6	ROUND_DOWN(USHRT_MAX - sizeof(struct tcp6_l2_head), 4)
>
> +#define MSS				(USHRT_MAX - sizeof(struct tcphdr))

We can't exceed USHRT_MAX at Layer-2 level, though, so the MSS for IPv4
should now be:

#define MSS4	ROUND_DOWN(USHRT_MAX - sizeof(struct ethhdr) -
			   	       sizeof(struct iphdr) -
				       sizeof(struct tcphdr),
			   4)

...and similar for IPv6.

>  #define WINDOW_DEFAULT			14600		/* RFC 6928 */
>  #ifdef HAS_SND_WND
> @@ -445,133 +413,78 @@ struct tcp_buf_seq_update {
>  };
>  
>  /* Static buffers */
> -
>  /**
> - * tcp4_l2_buf_t - Pre-cooked IPv4 packet buffers for tap connections
> - * @pad:	Align TCP header to 32 bytes, for AVX2 checksum calculation only
> - * @taph:	Tap-level headers (partially pre-filled)
> - * @iph:	Pre-filled IP header (except for tot_len and saddr)
> - * @uh:		Headroom for TCP header
> - * @data:	Storage for TCP payload
> + * tcp_l2_flags_t - TCP header and data to send option flags

'struct tcp_l2_flags_t - ...', by the way (pre-existing, I see).

> + * @th:		TCP header
> + * @opts	TCP option flags
>   */
> -static struct tcp4_l2_buf_t {
> -#ifdef __AVX2__
> -	uint8_t pad[26];	/* 0, align th to 32 bytes */
> -#else
> -	uint8_t pad[2];		/*	align iph to 4 bytes	0 */
> -#endif
> -	struct tap_hdr taph;	/* 26				2 */
> -	struct iphdr iph;	/* 44				20 */
> -	struct tcphdr th;	/* 64				40 */
> -	uint8_t data[MSS4];	/* 84				60 */
> -				/* 65536			65532 */
> +struct tcp_l2_flags_t {
> +	struct tcphdr th;
> +	char opts[OPT_MSS_LEN + OPT_WS_LEN + 1];
> +};

This should still be aligned (when declared below) with:

#ifdef __AVX2__
} __attribute__ ((packed, aligned(32)))
#else
} __attribute__ ((packed, aligned(__alignof__(unsigned int))))
#endif

because yes, now you get the unsigned int alignment for free, but the
32-byte boundary for AVX2 (checksums) not really, so that needs to be
there, and then for clarity I would keep the explicit attribute for the
non-AVX2 case.

By the way, I guess you could still combine the definition and
declaration as it was done earlier.

> +/**
> + * tcp_l2_payload_t - TCP header and data to send data
> + * 		32 bytes aligned to be able to use AVX2 checksum
> + * @th:		TCP header
> + * @data:	TCP data
> + */
> +struct tcp_l2_payload_t {
> +	struct tcphdr th;	/*    20 bytes */
> +	uint8_t data[MSS];	/* 65516 bytes */
>  #ifdef __AVX2__
> -} __attribute__ ((packed, aligned(32)))
> +} __attribute__ ((packed, aligned(32)));
>  #else
> -} __attribute__ ((packed, aligned(__alignof__(unsigned int))))
> +} __attribute__ ((packed, aligned(__alignof__(unsigned int))));
>  #endif
> -tcp4_l2_buf[TCP_FRAMES_MEM];
>  
> -static struct tcp_buf_seq_update tcp4_l2_buf_seq_update[TCP_FRAMES_MEM];
> +/* Ethernet header for IPv4 frames */
> +static struct ethhdr		tcp4_eth_src;
>  
> +/* IPv4 headers */
> +static struct iphdr		tcp4_l2_ip[TCP_FRAMES_MEM];
> +/* TCP headers and data for IPv4 frames */
> +static struct tcp_l2_payload_t	tcp4_l2_payload[TCP_FRAMES_MEM];
> +
> +static struct tcp_buf_seq_update tcp4_l2_buf_seq_update[TCP_FRAMES_MEM];
>  static unsigned int tcp4_l2_buf_used;
>  
> -/**
> - * tcp6_l2_buf_t - Pre-cooked IPv6 packet buffers for tap connections
> - * @pad:	Align IPv6 header for checksum calculation to 32B (AVX2) or 4B
> - * @taph:	Tap-level headers (partially pre-filled)
> - * @ip6h:	Pre-filled IP header (except for payload_len and addresses)
> - * @th:		Headroom for TCP header
> - * @data:	Storage for TCP payload
> - */
> -struct tcp6_l2_buf_t {
> -#ifdef __AVX2__
> -	uint8_t pad[14];	/* 0	align ip6h to 32 bytes */
> -#else
> -	uint8_t pad[2];		/*	align ip6h to 4 bytes	0 */
> -#endif
> -	struct tap_hdr taph;	/* 14				2 */
> -	struct ipv6hdr ip6h;	/* 32				20 */
> -	struct tcphdr th;	/* 72				60 */
> -	uint8_t data[MSS6];	/* 92				80 */
> -				/* 65536			65532 */
> -#ifdef __AVX2__
> -} __attribute__ ((packed, aligned(32)))
> -#else
> -} __attribute__ ((packed, aligned(__alignof__(unsigned int))))
> -#endif
> -tcp6_l2_buf[TCP_FRAMES_MEM];
> +/* IPv4 headers for TCP option flags frames */
> +static struct iphdr		tcp4_l2_flags_ip[TCP_FRAMES_MEM];
> +/* TCP headers and option flags for IPv4 frames */
> +static struct tcp_l2_flags_t	tcp4_l2_flags[TCP_FRAMES_MEM];
>  
> -static struct tcp_buf_seq_update tcp6_l2_buf_seq_update[TCP_FRAMES_MEM];
> +static unsigned int tcp4_l2_flags_buf_used;
> +
> +/* Ethernet header for IPv6 frames */
> +static struct ethhdr		tcp6_eth_src;
>  
> +/* IPv6 headers */
> +static struct ipv6hdr		tcp6_l2_ip[TCP_FRAMES_MEM];
> +/* TCP headers and data for IPv6 frames */
> +static struct tcp_l2_payload_t	tcp6_l2_payload[TCP_FRAMES_MEM];
> +
> +static struct tcp_buf_seq_update tcp6_l2_buf_seq_update[TCP_FRAMES_MEM];
>  static unsigned int tcp6_l2_buf_used;
>  
> +/* IPv6 headers for TCP option flags frames */
> +static struct ipv6hdr		tcp6_l2_flags_ip[TCP_FRAMES_MEM];
> +/* TCP headers and option flags for IPv6 frames */
> +static struct tcp_l2_flags_t	tcp6_l2_flags[TCP_FRAMES_MEM];
> +
> +static unsigned int tcp6_l2_flags_buf_used;
> +
>  /* recvmsg()/sendmsg() data for tap */
>  static char 		tcp_buf_discard		[MAX_WINDOW];
>  static struct iovec	iov_sock		[TCP_FRAMES_MEM + 1];
>  
> -static struct iovec	tcp4_l2_iov		[TCP_FRAMES_MEM];
> -static struct iovec	tcp6_l2_iov		[TCP_FRAMES_MEM];
> -static struct iovec	tcp4_l2_flags_iov	[TCP_FRAMES_MEM];
> -static struct iovec	tcp6_l2_flags_iov	[TCP_FRAMES_MEM];
> +static struct iovec	tcp4_l2_iov		[TCP_FRAMES_MEM][TCP_IOV_NUM];
> +static struct iovec	tcp6_l2_iov		[TCP_FRAMES_MEM][TCP_IOV_NUM];
> +static struct iovec	tcp4_l2_flags_iov	[TCP_FRAMES_MEM][TCP_IOV_NUM];
> +static struct iovec	tcp6_l2_flags_iov	[TCP_FRAMES_MEM][TCP_IOV_NUM];
>  
>  /* sendmsg() to socket */
>  static struct iovec	tcp_iov			[UIO_MAXIOV];
>  
> -/**
> - * tcp4_l2_flags_buf_t - IPv4 packet buffers for segments without data (flags)
> - * @pad:	Align TCP header to 32 bytes, for AVX2 checksum calculation only
> - * @taph:	Tap-level headers (partially pre-filled)
> - * @iph:	Pre-filled IP header (except for tot_len and saddr)
> - * @th:		Headroom for TCP header
> - * @opts:	Headroom for TCP options
> - */
> -static struct tcp4_l2_flags_buf_t {
> -#ifdef __AVX2__
> -	uint8_t pad[26];	/* 0, align th to 32 bytes */
> -#else
> -	uint8_t pad[2];		/*	align iph to 4 bytes	0 */
> -#endif
> -	struct tap_hdr taph;	/* 26				2 */
> -	struct iphdr iph;	/* 44				20 */
> -	struct tcphdr th;	/* 64				40 */
> -	char opts[OPT_MSS_LEN + OPT_WS_LEN + 1];
> -#ifdef __AVX2__
> -} __attribute__ ((packed, aligned(32)))
> -#else
> -} __attribute__ ((packed, aligned(__alignof__(unsigned int))))
> -#endif
> -tcp4_l2_flags_buf[TCP_FRAMES_MEM];
> -
> -static unsigned int tcp4_l2_flags_buf_used;
> -
> -/**
> - * tcp6_l2_flags_buf_t - IPv6 packet buffers for segments without data (flags)
> - * @pad:	Align IPv6 header for checksum calculation to 32B (AVX2) or 4B
> - * @taph:	Tap-level headers (partially pre-filled)
> - * @ip6h:	Pre-filled IP header (except for payload_len and addresses)
> - * @th:		Headroom for TCP header
> - * @opts:	Headroom for TCP options
> - */
> -static struct tcp6_l2_flags_buf_t {
> -#ifdef __AVX2__
> -	uint8_t pad[14];	/* 0	align ip6h to 32 bytes */
> -#else
> -	uint8_t pad[2];		/*	align ip6h to 4 bytes		   0 */
> -#endif
> -	struct tap_hdr taph;	/* 14					   2 */
> -	struct ipv6hdr ip6h;	/* 32					  20 */
> -	struct tcphdr th	/* 72 */ __attribute__ ((aligned(4))); /* 60 */
> -	char opts[OPT_MSS_LEN + OPT_WS_LEN + 1];
> -#ifdef __AVX2__
> -} __attribute__ ((packed, aligned(32)))
> -#else
> -} __attribute__ ((packed, aligned(__alignof__(unsigned int))))
> -#endif
> -tcp6_l2_flags_buf[TCP_FRAMES_MEM];
> -
> -static unsigned int tcp6_l2_flags_buf_used;
> -
>  #define CONN(idx)		(&(FLOW(idx)->tcp))
>  
>  /* Table for lookup from remote address, local port, remote port */
> @@ -973,19 +886,8 @@ static void tcp_update_check_tcp6(struct ipv6hdr *ip6h, struct tcphdr *th)
>   */
>  void tcp_update_l2_buf(const unsigned char *eth_d, const unsigned char *eth_s)

This doesn't update "L2 buffers" anymore, just the Ethernet addresses
(function comment should be updated).

>  {
> -	int i;
> -
> -	for (i = 0; i < TCP_FRAMES_MEM; i++) {
> -		struct tcp4_l2_flags_buf_t *b4f = &tcp4_l2_flags_buf[i];
> -		struct tcp6_l2_flags_buf_t *b6f = &tcp6_l2_flags_buf[i];
> -		struct tcp4_l2_buf_t *b4 = &tcp4_l2_buf[i];
> -		struct tcp6_l2_buf_t *b6 = &tcp6_l2_buf[i];
> -
> -		eth_update_mac(&b4->taph.eh, eth_d, eth_s);
> -		eth_update_mac(&b6->taph.eh, eth_d, eth_s);
> -		eth_update_mac(&b4f->taph.eh, eth_d, eth_s);
> -		eth_update_mac(&b6f->taph.eh, eth_d, eth_s);
> -	}
> +	eth_update_mac(&tcp4_eth_src, eth_d, eth_s);
> +	eth_update_mac(&tcp6_eth_src, eth_d, eth_s);
>  }
>  
>  /**
> @@ -995,29 +897,42 @@ void tcp_update_l2_buf(const unsigned char *eth_d, const unsigned char *eth_s)
>  static void tcp_sock4_iov_init(const struct ctx *c)
>  {
>  	struct iphdr iph = L2_BUF_IP4_INIT(IPPROTO_TCP);
> -	struct iovec *iov;
>  	int i;
>  
> -	for (i = 0; i < ARRAY_SIZE(tcp4_l2_buf); i++) {
> -		tcp4_l2_buf[i] = (struct tcp4_l2_buf_t) {
> -			.taph = TAP_HDR_INIT(ETH_P_IP),
> -			.iph = iph,
> -			.th = { .doff = sizeof(struct tcphdr) / 4, .ack = 1 }
> -		};
> -	}
> +	(void)c;

...then drop it from the parameter list?

>  
> -	for (i = 0; i < ARRAY_SIZE(tcp4_l2_flags_buf); i++) {
> -		tcp4_l2_flags_buf[i] = (struct tcp4_l2_flags_buf_t) {
> -			.taph = TAP_HDR_INIT(ETH_P_IP),
> -			.iph = L2_BUF_IP4_INIT(IPPROTO_TCP)
> -		};
> -	}
> +	tcp4_eth_src.h_proto = htons_constant(ETH_P_IP);
> +	for (i = 0; i < TCP_FRAMES_MEM; i++) {
> +		struct iovec *iov;
> +
> +		/* headers */
> +		tcp4_l2_ip[i] = iph;
> +		tcp4_l2_payload[i].th = (struct tcphdr){
> +					.doff = sizeof(struct tcphdr) / 4,
> +					.ack = 1
> +				};
>  
> -	for (i = 0, iov = tcp4_l2_iov; i < TCP_FRAMES_MEM; i++, iov++)
> -		iov->iov_base = tap_iov_base(c, &tcp4_l2_buf[i].taph);
> +		tcp4_l2_flags_ip[i] = iph;
> +		tcp4_l2_flags[i].th = (struct tcphdr){
> +					.doff = sizeof(struct tcphdr) / 4,
> +					.ack = 1
> +				};
>  
> -	for (i = 0, iov = tcp4_l2_flags_iov; i < TCP_FRAMES_MEM; i++, iov++)
> -		iov->iov_base = tap_iov_base(c, &tcp4_l2_flags_buf[i].taph);
> +		/* iovecs */
> +		iov = tcp4_l2_iov[i];
> +		iov[TCP_IOV_ETH].iov_base = &tcp4_eth_src;
> +		iov[TCP_IOV_ETH].iov_len = sizeof(struct ethhdr);
> +		iov[TCP_IOV_IP].iov_base = &tcp4_l2_ip[i];
> +		iov[TCP_IOV_IP].iov_len = sizeof(struct iphdr);
> +		iov[TCP_IOV_PAYLOAD].iov_base = &tcp4_l2_payload[i];
> +
> +		iov = tcp4_l2_flags_iov[i];
> +		iov[TCP_IOV_ETH].iov_base = &tcp4_eth_src;
> +		iov[TCP_IOV_ETH].iov_len = sizeof(struct ethhdr);
> +		iov[TCP_IOV_IP].iov_base = &tcp4_l2_flags_ip[i];
> +		iov[TCP_IOV_IP].iov_len = sizeof(struct iphdr);
> +		iov[TCP_IOV_PAYLOAD].iov_base = &tcp4_l2_flags[i];
> +	}
>  }
>  
>  /**
> @@ -1026,29 +941,43 @@ static void tcp_sock4_iov_init(const struct ctx *c)
>   */
>  static void tcp_sock6_iov_init(const struct ctx *c)
>  {
> -	struct iovec *iov;
> +	struct ipv6hdr ip6 = L2_BUF_IP6_INIT(IPPROTO_TCP);
>  	int i;
>  
> -	for (i = 0; i < ARRAY_SIZE(tcp6_l2_buf); i++) {
> -		tcp6_l2_buf[i] = (struct tcp6_l2_buf_t) {
> -			.taph = TAP_HDR_INIT(ETH_P_IPV6),
> -			.ip6h = L2_BUF_IP6_INIT(IPPROTO_TCP),
> -			.th = { .doff = sizeof(struct tcphdr) / 4, .ack = 1 }
> -		};
> -	}
> +	(void)c;
>  
> -	for (i = 0; i < ARRAY_SIZE(tcp6_l2_flags_buf); i++) {
> -		tcp6_l2_flags_buf[i] = (struct tcp6_l2_flags_buf_t) {
> -			.taph = TAP_HDR_INIT(ETH_P_IPV6),
> -			.ip6h = L2_BUF_IP6_INIT(IPPROTO_TCP)
> -		};
> -	}
> +	tcp6_eth_src.h_proto = htons_constant(ETH_P_IPV6);
> +	for (i = 0; i < TCP_FRAMES_MEM; i++) {
> +		struct iovec *iov;
> +
> +		/* headers */
> +		tcp6_l2_ip[i] = ip6;
> +		tcp6_l2_payload[i].th = (struct tcphdr){
> +					.doff = sizeof(struct tcphdr) / 4,
> +					.ack = 1
> +				};
>  
> -	for (i = 0, iov = tcp6_l2_iov; i < TCP_FRAMES_MEM; i++, iov++)
> -		iov->iov_base = tap_iov_base(c, &tcp6_l2_buf[i].taph);
> +		tcp6_l2_flags_ip[i] = ip6;
> +		tcp6_l2_flags[i].th = (struct tcphdr){
> +					.doff = sizeof(struct tcphdr) / 4,
> +					.ack = 1
> +				};
>  
> -	for (i = 0, iov = tcp6_l2_flags_iov; i < TCP_FRAMES_MEM; i++, iov++)
> -		iov->iov_base = tap_iov_base(c, &tcp6_l2_flags_buf[i].taph);
> +		/* iovecs */
> +		iov = tcp6_l2_iov[i];
> +		iov[TCP_IOV_ETH].iov_base = &tcp6_eth_src;
> +		iov[TCP_IOV_ETH].iov_len = sizeof(struct ethhdr);
> +		iov[TCP_IOV_IP].iov_base = &tcp6_l2_ip[i];
> +		iov[TCP_IOV_IP].iov_len = sizeof(struct ipv6hdr);
> +		iov[TCP_IOV_PAYLOAD].iov_base = &tcp6_l2_payload[i];
> +
> +		iov = tcp6_l2_flags_iov[i];
> +		iov[TCP_IOV_ETH].iov_base = &tcp6_eth_src;
> +		iov[TCP_IOV_ETH].iov_len = sizeof(struct ethhdr);
> +		iov[TCP_IOV_IP].iov_base = &tcp6_l2_flags_ip[i];
> +		iov[TCP_IOV_IP].iov_len = sizeof(struct ipv6hdr);
> +		iov[TCP_IOV_PAYLOAD].iov_base = &tcp6_l2_flags[i];
> +	}
>  }
>  
>  /**
> @@ -1289,10 +1218,10 @@ static void tcp_rst_do(struct ctx *c, struct tcp_tap_conn *conn);
>   */
>  static void tcp_l2_flags_buf_flush(const struct ctx *c)
>  {
> -	tap_send_frames(c, tcp6_l2_flags_iov, tcp6_l2_flags_buf_used);
> +	tap_send_iov(c, tcp6_l2_flags_iov, tcp6_l2_flags_buf_used);
>  	tcp6_l2_flags_buf_used = 0;
>  
> -	tap_send_frames(c, tcp4_l2_flags_iov, tcp4_l2_flags_buf_used);
> +	tap_send_iov(c, tcp4_l2_flags_iov, tcp4_l2_flags_buf_used);
>  	tcp4_l2_flags_buf_used = 0;
>  }
>  
> @@ -1305,12 +1234,12 @@ static void tcp_l2_data_buf_flush(const struct ctx *c)
>  	unsigned i;
>  	size_t m;
>  
> -	m = tap_send_frames(c, tcp6_l2_iov, tcp6_l2_buf_used);
> +	m = tap_send_iov(c, tcp6_l2_iov, tcp6_l2_buf_used);
>  	for (i = 0; i < m; i++)
>  		*tcp6_l2_buf_seq_update[i].seq += tcp6_l2_buf_seq_update[i].len;
>  	tcp6_l2_buf_used = 0;
>  
> -	m = tap_send_frames(c, tcp4_l2_iov, tcp4_l2_buf_used);
> +	m = tap_send_iov(c, tcp4_l2_iov, tcp4_l2_buf_used);
>  	for (i = 0; i < m; i++)
>  		*tcp4_l2_buf_seq_update[i].seq += tcp4_l2_buf_seq_update[i].len;
>  	tcp4_l2_buf_used = 0;
> @@ -1433,7 +1362,7 @@ static size_t tcp_fill_headers6(const struct ctx *c,
>   * tcp_l2_buf_fill_headers() - Fill 802.3, IP, TCP headers in pre-cooked buffers
>   * @c:		Execution context
>   * @conn:	Connection pointer
> - * @p:		Pointer to any type of TCP pre-cooked buffer
> + * @iov:	Pointer to an array of iovec of TCP pre-cooked buffer
>   * @plen:	Payload length (including TCP header options)
>   * @check:	Checksum, if already known
>   * @seq:	Sequence number for this segment
> @@ -1442,26 +1371,22 @@ static size_t tcp_fill_headers6(const struct ctx *c,
>   */
>  static size_t tcp_l2_buf_fill_headers(const struct ctx *c,
>  				      const struct tcp_tap_conn *conn,
> -				      void *p, size_t plen,
> +				      struct iovec *iov, size_t plen,
>  				      const uint16_t *check, uint32_t seq)
>  {
>  	const struct in_addr *a4 = inany_v4(&conn->faddr);
>  	size_t ip_len, tlen;
>  
>  	if (a4) {
> -		struct tcp4_l2_buf_t *b = (struct tcp4_l2_buf_t *)p;
> -
> -		ip_len = tcp_fill_headers4(c, conn, &b->iph, &b->th, plen,
> +		ip_len = tcp_fill_headers4(c, conn, iov[TCP_IOV_IP].iov_base,
> +					   iov[TCP_IOV_PAYLOAD].iov_base, plen,
>  					   check, seq);
> -
> -		tlen = tap_iov_len(c, &b->taph, ip_len);
> +		tlen = ip_len - sizeof(struct iphdr);
>  	} else {
> -		struct tcp6_l2_buf_t *b = (struct tcp6_l2_buf_t *)p;
> -
> -		ip_len = tcp_fill_headers6(c, conn, &b->ip6h, &b->th, plen,
> +		ip_len = tcp_fill_headers6(c, conn, iov[TCP_IOV_IP].iov_base,
> +					   iov[TCP_IOV_PAYLOAD].iov_base, plen,
>  					   seq);
> -
> -		tlen = tap_iov_len(c, &b->taph, ip_len);
> +		tlen = ip_len - sizeof(struct ipv6hdr);
>  	}
>  
>  	return tlen;
> @@ -1595,16 +1520,15 @@ static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
>  {
>  	uint32_t prev_ack_to_tap = conn->seq_ack_to_tap;
>  	uint32_t prev_wnd_to_tap = conn->wnd_to_tap;
> -	struct tcp4_l2_flags_buf_t *b4 = NULL;
> -	struct tcp6_l2_flags_buf_t *b6 = NULL;
> +	struct tcp_l2_flags_t *payload;
>  	struct tcp_info tinfo = { 0 };
>  	socklen_t sl = sizeof(tinfo);
>  	int s = conn->sock;
>  	size_t optlen = 0;
>  	struct iovec *iov;
> +	struct iovec *dup_iov;

This should now go before 'int s' (longest to shortest), or the
declaration could be combined on the same line (it's really symmetric)
with *iov.

>  	struct tcphdr *th;
>  	char *data;
> -	void *p;
>  
>  	if (SEQ_GE(conn->seq_ack_to_tap, conn->seq_from_tap) &&
>  	    !flags && conn->wnd_to_tap)
> @@ -1627,18 +1551,15 @@ static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
>  		return 0;
>  
>  	if (CONN_V4(conn)) {
> -		iov = tcp4_l2_flags_iov    + tcp4_l2_flags_buf_used;
> -		p = b4 = tcp4_l2_flags_buf + tcp4_l2_flags_buf_used++;
> -		th = &b4->th;
> -
> -		/* gcc 11.2 would complain on data = (char *)(th + 1); */
> -		data = b4->opts;
> +		iov = tcp4_l2_flags_iov[tcp4_l2_flags_buf_used++];
> +		dup_iov = tcp4_l2_flags_iov[tcp4_l2_flags_buf_used];
>  	} else {
> -		iov = tcp6_l2_flags_iov    + tcp6_l2_flags_buf_used;
> -		p = b6 = tcp6_l2_flags_buf + tcp6_l2_flags_buf_used++;
> -		th = &b6->th;
> -		data = b6->opts;
> +		iov = tcp6_l2_flags_iov[tcp6_l2_flags_buf_used++];
> +		dup_iov = tcp6_l2_flags_iov[tcp6_l2_flags_buf_used];

Maybe you could move the dup_iov assignments below, and group these
with the increments of tcp[46]_l2_flags_buf_used, otherwise I'll scream
"off-by-one" every time I see this (even though it's not actually the case).

>  	}
> +	payload = iov[TCP_IOV_PAYLOAD].iov_base;
> +	th = &payload->th;
> +	data = payload->opts;
>  
>  	if (flags & SYN) {
>  		int mss;
> @@ -1653,10 +1574,6 @@ static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
>  			mss = tinfo.tcpi_snd_mss;
>  		} else {
>  			mss = c->mtu - sizeof(struct tcphdr);
> -			if (CONN_V4(conn))
> -				mss -= sizeof(struct iphdr);
> -			else
> -				mss -= sizeof(struct ipv6hdr);
>  
>  			if (c->low_wmem &&
>  			    !(conn->flags & LOCAL) && !tcp_rtt_dst_low(conn))
> @@ -1688,8 +1605,9 @@ static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
>  	th->syn = !!(flags & SYN);
>  	th->fin = !!(flags & FIN);
>  
> -	iov->iov_len = tcp_l2_buf_fill_headers(c, conn, p, optlen,
> -					       NULL, conn->seq_to_tap);
> +	iov[TCP_IOV_PAYLOAD].iov_len = tcp_l2_buf_fill_headers(c, conn, iov,
> +							       optlen, NULL,
> +							      conn->seq_to_tap);
>  
>  	if (th->ack) {
>  		if (SEQ_GE(conn->seq_ack_to_tap, conn->seq_from_tap))
> @@ -1705,23 +1623,26 @@ static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
>  	if (th->fin || th->syn)
>  		conn->seq_to_tap++;
>  
> +	if (flags & DUP_ACK) {
> +		int i;
> +		for (i = 0; i < TCP_IOV_NUM; i++) {
> +			memcpy(dup_iov[i].iov_base, iov[i].iov_base,
> +			       iov[i].iov_len);
> +			dup_iov[i].iov_len = iov[i].iov_len;
> +		}
> +	}
> +
>  	if (CONN_V4(conn)) {
> -		if (flags & DUP_ACK) {
> -			memcpy(b4 + 1, b4, sizeof(*b4));
> -			(iov + 1)->iov_len = iov->iov_len;
> +		if (flags & DUP_ACK)
>  			tcp4_l2_flags_buf_used++;
> -		}
>  
> -		if (tcp4_l2_flags_buf_used > ARRAY_SIZE(tcp4_l2_flags_buf) - 2)
> +		if (tcp4_l2_flags_buf_used > TCP_FRAMES_MEM - 2)
>  			tcp_l2_flags_buf_flush(c);
>  	} else {
> -		if (flags & DUP_ACK) {
> -			memcpy(b6 + 1, b6, sizeof(*b6));
> -			(iov + 1)->iov_len = iov->iov_len;
> +		if (flags & DUP_ACK)
>  			tcp6_l2_flags_buf_used++;
> -		}
>  
> -		if (tcp6_l2_flags_buf_used > ARRAY_SIZE(tcp6_l2_flags_buf) - 2)
> +		if (tcp6_l2_flags_buf_used > TCP_FRAMES_MEM - 2)
>  			tcp_l2_flags_buf_flush(c);
>  	}
>  
> @@ -1887,15 +1808,14 @@ static uint16_t tcp_conn_tap_mss(const struct tcp_tap_conn *conn,
>  	unsigned int mss;
>  	int ret;
>  
> +	(void)conn; /* unused */
> +
>  	if ((ret = tcp_opt_get(opts, optlen, OPT_MSS, NULL, NULL)) < 0)
>  		mss = MSS_DEFAULT;
>  	else
>  		mss = ret;
>  
> -	if (CONN_V4(conn))
> -		mss = MIN(MSS4, mss);
> -	else
> -		mss = MIN(MSS6, mss);
> +	mss = MIN(MSS, mss);
>  
>  	return MIN(mss, USHRT_MAX);
>  }
> @@ -2171,27 +2091,30 @@ static void tcp_data_to_tap(const struct ctx *c, struct tcp_tap_conn *conn,
>  	struct iovec *iov;
>  
>  	if (CONN_V4(conn)) {
> -		struct tcp4_l2_buf_t *b = &tcp4_l2_buf[tcp4_l2_buf_used];
> -		const uint16_t *check = no_csum ? &(b - 1)->iph.check : NULL;
> +		struct iovec *iov_prev = tcp4_l2_iov[tcp4_l2_buf_used - 1];
> +		const uint16_t *check = NULL;
> +
> +		if (no_csum) {
> +			struct iphdr *iph = iov_prev[TCP_IOV_IP].iov_base;
> +			check = &iph->check;
> +		}
>  
>  		tcp4_l2_buf_seq_update[tcp4_l2_buf_used].seq = seq_update;
>  		tcp4_l2_buf_seq_update[tcp4_l2_buf_used].len = plen;
>  
> -		iov = tcp4_l2_iov + tcp4_l2_buf_used++;
> -		iov->iov_len = tcp_l2_buf_fill_headers(c, conn, b, plen,
> -						       check, seq);
> -		if (tcp4_l2_buf_used > ARRAY_SIZE(tcp4_l2_buf) - 1)
> +		iov = tcp4_l2_iov[tcp4_l2_buf_used++];
> +		iov[TCP_IOV_PAYLOAD].iov_len = tcp_l2_buf_fill_headers(c, conn,
> +							iov, plen, check, seq);
> +		if (tcp4_l2_buf_used > TCP_FRAMES_MEM - 1)
>  			tcp_l2_data_buf_flush(c);
>  	} else if (CONN_V6(conn)) {
> -		struct tcp6_l2_buf_t *b = &tcp6_l2_buf[tcp6_l2_buf_used];
> -
>  		tcp6_l2_buf_seq_update[tcp6_l2_buf_used].seq = seq_update;
>  		tcp6_l2_buf_seq_update[tcp6_l2_buf_used].len = plen;
>  
> -		iov = tcp6_l2_iov + tcp6_l2_buf_used++;
> -		iov->iov_len = tcp_l2_buf_fill_headers(c, conn, b, plen,
> -						       NULL, seq);
> -		if (tcp6_l2_buf_used > ARRAY_SIZE(tcp6_l2_buf) - 1)
> +		iov = tcp6_l2_iov[tcp6_l2_buf_used++];
> +		iov[TCP_IOV_PAYLOAD].iov_len = tcp_l2_buf_fill_headers(c, conn,
> +							iov, plen, NULL, seq);

Maybe use a temporary variable?

> +		if (tcp6_l2_buf_used > TCP_FRAMES_MEM - 1)
>  			tcp_l2_data_buf_flush(c);
>  	}
>  }
> @@ -2247,8 +2170,8 @@ static int tcp_data_from_sock(struct ctx *c, struct tcp_tap_conn *conn)
>  	iov_sock[0].iov_base = tcp_buf_discard;
>  	iov_sock[0].iov_len = already_sent;
>  
> -	if (( v4 && tcp4_l2_buf_used + fill_bufs > ARRAY_SIZE(tcp4_l2_buf)) ||
> -	    (!v4 && tcp6_l2_buf_used + fill_bufs > ARRAY_SIZE(tcp6_l2_buf))) {
> +	if (( v4 && tcp4_l2_buf_used + fill_bufs > TCP_FRAMES_MEM) ||
> +	    (!v4 && tcp6_l2_buf_used + fill_bufs > TCP_FRAMES_MEM)) {
>  		tcp_l2_data_buf_flush(c);
>  
>  		/* Silence Coverity CWE-125 false positive */
> @@ -2257,9 +2180,9 @@ static int tcp_data_from_sock(struct ctx *c, struct tcp_tap_conn *conn)
>  
>  	for (i = 0, iov = iov_sock + 1; i < fill_bufs; i++, iov++) {
>  		if (v4)
> -			iov->iov_base = &tcp4_l2_buf[tcp4_l2_buf_used + i].data;
> +			iov->iov_base = &tcp4_l2_payload[tcp4_l2_buf_used + i].data;
>  		else
> -			iov->iov_base = &tcp6_l2_buf[tcp6_l2_buf_used + i].data;
> +			iov->iov_base = &tcp6_l2_payload[tcp6_l2_buf_used + i].data;
>  		iov->iov_len = mss;
>  	}
>  	if (iov_rem)

The rest looks good to me (at the moment :)).

-- 
Stefano