[PATCH 11/24] tcp: move buffers management functions to their own file

[Laurent Vivier <lvivier@redhat.com>]

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---
 Makefile       |   7 +-
 tcp.c          | 618 ++-----------------------------------------------
 tcp_buf.c      | 569 +++++++++++++++++++++++++++++++++++++++++++++
 tcp_buf.h      |  17 ++
 tcp_internal.h |  78 +++++++
 5 files changed, 689 insertions(+), 600 deletions(-)
 create mode 100644 tcp_buf.c
 create mode 100644 tcp_buf.h
 create mode 100644 tcp_internal.h

diff --git a/Makefile b/Makefile
index acf37f5a2036..bf370b6ec2e6 100644
--- a/Makefile
+++ b/Makefile
@@ -46,8 +46,8 @@ FLAGS += -DDUAL_STACK_SOCKETS=$(DUAL_STACK_SOCKETS)
 
 PASST_SRCS = arch.c arp.c checksum.c conf.c dhcp.c dhcpv6.c flow.c icmp.c \
 	igmp.c isolation.c lineread.c log.c mld.c ndp.c netlink.c packet.c \
-	passt.c pasta.c pcap.c pif.c port_fwd.c tap.c tcp.c tcp_splice.c udp.c \
-	util.c iov.c ip.c
+	passt.c pasta.c pcap.c pif.c port_fwd.c tap.c tcp.c tcp_splice.c \
+	tcp_buf.c udp.c util.c iov.c ip.c
 QRAP_SRCS = qrap.c
 SRCS = $(PASST_SRCS) $(QRAP_SRCS)
 
@@ -56,7 +56,8 @@ MANPAGES = passt.1 pasta.1 qrap.1
 PASST_HEADERS = arch.h arp.h checksum.h conf.h dhcp.h dhcpv6.h flow.h \
 	flow_table.h icmp.h inany.h isolation.h lineread.h log.h ndp.h \
 	netlink.h packet.h passt.h pasta.h pcap.h pif.h port_fwd.h siphash.h \
-	tap.h tcp.h tcp_conn.h tcp_splice.h udp.h util.h iov.h ip.h
+	tap.h tcp.h tcp_conn.h tcp_splice.h tcp_buf.h tcp_internal.h udp.h \
+	util.h iov.h ip.h
 HEADERS = $(PASST_HEADERS) seccomp.h
 
 C := \#include <linux/tcp.h>\nstruct tcp_info x = { .tcpi_snd_wnd = 0 };
diff --git a/tcp.c b/tcp.c
index 640209533772..54c15087d678 100644
--- a/tcp.c
+++ b/tcp.c
@@ -300,57 +300,19 @@
 #include "flow.h"
 
 #include "flow_table.h"
+#include "tcp_internal.h"
+#include "tcp_buf.h"
 
 /* Sides of a flow as we use them in "tap" connections */
 #define	SOCKSIDE	0
 #define	TAPSIDE		1
 
-#define TCP_FRAMES_MEM			128
-#define TCP_FRAMES							\
-	(c->mode == MODE_PASST ? TCP_FRAMES_MEM : 1)
-
 #define TCP_HASH_TABLE_LOAD		70		/* % */
 #define TCP_HASH_TABLE_SIZE		(FLOW_MAX * 100 / TCP_HASH_TABLE_LOAD)
 
-#define MAX_WS				8
-#define MAX_WINDOW			(1 << (16 + (MAX_WS)))
-
 /* 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
-
-#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 WINDOW_DEFAULT			14600		/* RFC 6928 */
 #ifdef HAS_SND_WND
 # define KERNEL_REPORTS_SND_WND(c)	(c->tcp.kernel_snd_wnd)
@@ -372,31 +334,9 @@ struct tcp6_l2_head {	/* For MSS6 macro: keep in sync with tcp6_l2_buf_t */
  */
 #define SOL_TCP				IPPROTO_TCP
 
-#define SEQ_LE(a, b)			((b) - (a) < MAX_WINDOW)
-#define SEQ_LT(a, b)			((b) - (a) - 1 < MAX_WINDOW)
-#define SEQ_GE(a, b)			((a) - (b) < MAX_WINDOW)
-#define SEQ_GT(a, b)			((a) - (b) - 1 < MAX_WINDOW)
-
-#define FIN		(1 << 0)
-#define SYN		(1 << 1)
-#define RST		(1 << 2)
-#define ACK		(1 << 4)
-/* Flags for internal usage */
-#define DUP_ACK		(1 << 5)
 #define ACK_IF_NEEDED	0		/* See tcp_buf_send_flag() */
 
-#define OPT_EOL		0
-#define OPT_NOP		1
-#define OPT_MSS		2
-#define OPT_MSS_LEN	4
-#define OPT_WS		3
-#define OPT_WS_LEN	3
-#define OPT_SACKP	4
-#define OPT_SACK	5
-#define OPT_TS		8
-
-#define CONN_V4(conn)		(!!inany_v4(&(conn)->faddr))
-#define CONN_V6(conn)		(!CONN_V4(conn))
+
 #define CONN_IS_CLOSING(conn)						\
 	((conn->events & ESTABLISHED) &&				\
 	 (conn->events & (SOCK_FIN_RCVD | TAP_FIN_RCVD)))
@@ -433,144 +373,11 @@ 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 */
-
-/**
- * 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
- */
-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 */
-#ifdef __AVX2__
-} __attribute__ ((packed, aligned(32)))
-#else
-} __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];
-
-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];
-
-static struct tcp_buf_seq_update tcp6_l2_buf_seq_update[TCP_FRAMES_MEM];
-
-static unsigned int tcp6_l2_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];
+char		tcp_buf_discard		[MAX_WINDOW];
 
 /* 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 */
@@ -611,14 +418,6 @@ static uint32_t tcp_conn_epoll_events(uint8_t events, uint8_t conn_flags)
 	return EPOLLRDHUP;
 }
 
-static void conn_flag_do(const struct ctx *c, struct tcp_tap_conn *conn,
-			 unsigned long flag);
-#define conn_flag(c, conn, flag)					\
-	do {								\
-		flow_trace(conn, "flag at %s:%i", __func__, __LINE__);	\
-		conn_flag_do(c, conn, flag);				\
-	} while (0)
-
 /**
  * tcp_epoll_ctl() - Add/modify/delete epoll state from connection events
  * @c:		Execution context
@@ -730,8 +529,8 @@ static void tcp_timer_ctl(const struct ctx *c, struct tcp_tap_conn *conn)
  * @conn:	Connection pointer
  * @flag:	Flag to set, or ~flag to unset
  */
-static void conn_flag_do(const struct ctx *c, struct tcp_tap_conn *conn,
-			 unsigned long flag)
+void conn_flag_do(const struct ctx *c, struct tcp_tap_conn *conn,
+		  unsigned long flag)
 {
 	if (flag & (flag - 1)) {
 		int flag_index = fls(~flag);
@@ -781,8 +580,8 @@ static void tcp_hash_remove(const struct ctx *c,
  * @conn:	Connection pointer
  * @event:	Connection event
  */
-static void conn_event_do(const struct ctx *c, struct tcp_tap_conn *conn,
-			  unsigned long event)
+void conn_event_do(const struct ctx *c, struct tcp_tap_conn *conn,
+		   unsigned long event)
 {
 	int prev, new, num = fls(event);
 
@@ -830,12 +629,6 @@ static void conn_event_do(const struct ctx *c, struct tcp_tap_conn *conn,
 		tcp_timer_ctl(c, conn);
 }
 
-#define conn_event(c, conn, event)					\
-	do {								\
-		flow_trace(conn, "event at %s:%i", __func__, __LINE__);	\
-		conn_event_do(c, conn, event);				\
-	} while (0)
-
 /**
  * tcp_rtt_dst_low() - Check if low RTT was seen for connection endpoint
  * @conn:	Connection pointer
@@ -959,91 +752,6 @@ static uint16_t tcp_update_check_tcp6(struct ipv6hdr *ip6h)
 	return csum(th, ntohs(ip6h->payload_len), sum);
 }
 
-/**
- * tcp_buf_update_l2() - Update L2 buffers with Ethernet and IPv4 addresses
- * @eth_d:	Ethernet destination address, NULL if unchanged
- * @eth_s:	Ethernet source address, NULL if unchanged
- */
-void tcp_buf_update_l2(const unsigned char *eth_d, const unsigned char *eth_s)
-{
-	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];
-
-		tap_update_mac(&b4->taph, eth_d, eth_s);
-		tap_update_mac(&b6->taph, eth_d, eth_s);
-		tap_update_mac(&b4f->taph, eth_d, eth_s);
-		tap_update_mac(&b6f->taph, eth_d, eth_s);
-	}
-}
-
-/**
- * tcp_buf_sock4_iov_init() - Initialise scatter-gather L2 buffers for IPv4 sockets
- * @c:		Execution context
- */
-static void tcp_buf_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 }
-		};
-	}
-
-	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)
-		};
-	}
-
-	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);
-
-	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);
-}
-
-/**
- * tcp_buf_sock6_iov_init() - Initialise scatter-gather L2 buffers for IPv6 sockets
- * @c:		Execution context
- */
-static void tcp_buf_sock6_iov_init(const struct ctx *c)
-{
-	struct iovec *iov;
-	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 }
-		};
-	}
-
-	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)
-		};
-	}
-
-	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);
-
-	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);
-}
-
 /**
  * tcp_opt_get() - Get option, and value if any, from TCP header
  * @opts:	Pointer to start of TCP options in header
@@ -1269,46 +977,6 @@ bool tcp_flow_defer(union flow *flow)
 	return true;
 }
 
-static void tcp_rst_do(struct ctx *c, struct tcp_tap_conn *conn);
-#define tcp_rst(c, conn)						\
-	do {								\
-		flow_dbg((conn), "TCP reset at %s:%i", __func__, __LINE__); \
-		tcp_rst_do(c, conn);					\
-	} while (0)
-
-/**
- * tcp_buf_l2_flags_flush() - Send out buffers for segments with no data (flags)
- * @c:		Execution context
- */
-static void tcp_buf_l2_flags_flush(const struct ctx *c)
-{
-	tap_send_frames(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);
-	tcp4_l2_flags_buf_used = 0;
-}
-
-/**
- * tcp_buf_l2_data_flush() - Send out buffers for segments with data
- * @c:		Execution context
- */
-static void tcp_buf_l2_data_flush(const struct ctx *c)
-{
-	unsigned i;
-	size_t m;
-
-	m = tap_send_frames(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);
-	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;
-}
-
 /**
  * tcp_defer_handler() - Handler for TCP deferred tasks
  * @c:		Execution context
@@ -1348,10 +1016,10 @@ static void tcp_set_tcp_header(struct tcphdr *th,
  * Return: IP frame length including L2 headers, host order
  */
 
-static size_t ipv4_fill_headers(const struct ctx *c,
-				const struct tcp_tap_conn *conn,
-				struct iphdr *iph, size_t plen,
-				const uint16_t *check, uint32_t seq)
+size_t ipv4_fill_headers(const struct ctx *c,
+			 const struct tcp_tap_conn *conn,
+			 struct iphdr *iph, size_t plen,
+			 const uint16_t *check, uint32_t seq)
 {
 	struct tcphdr *th = (void *)(iph + 1);
 	const struct in_addr *a4 = inany_v4(&conn->faddr);
@@ -1382,10 +1050,10 @@ static size_t ipv4_fill_headers(const struct ctx *c,
  * Return: IP frame length including L2 headers, host order
  */
 
-static size_t ipv6_fill_headers(const struct ctx *c,
-				const struct tcp_tap_conn *conn,
-				struct ipv6hdr *ip6h, size_t plen,
-				uint32_t seq)
+size_t ipv6_fill_headers(const struct ctx *c,
+			 const struct tcp_tap_conn *conn,
+			 struct ipv6hdr *ip6h, size_t plen,
+			 uint32_t seq)
 {
 	struct tcphdr *th = (void *)(ip6h + 1);
 	size_t ip_len = plen + sizeof(struct ipv6hdr) + sizeof(struct tcphdr);
@@ -1423,8 +1091,8 @@ static size_t ipv6_fill_headers(const struct ctx *c,
  *
  * Return: 1 if sequence or window were updated, 0 otherwise
  */
-static int tcp_update_seqack_wnd(const struct ctx *c, struct tcp_tap_conn *conn,
-				 int force_seq, struct tcp_info *tinfo)
+int tcp_update_seqack_wnd(const struct ctx *c, struct tcp_tap_conn *conn,
+			  int force_seq, struct tcp_info *tinfo)
 {
 	uint32_t prev_wnd_to_tap = conn->wnd_to_tap << conn->ws_to_tap;
 	uint32_t prev_ack_to_tap = conn->seq_ack_to_tap;
@@ -1539,7 +1207,8 @@ static void tcp_update_seqack_from_tap(const struct ctx *c,
  * Return: negative error code on connection reset, 0 otherwise
  */
 
-static int do_tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags,			    struct tcphdr *th, char *data, size_t optlen)
+int do_tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags,
+		     struct tcphdr *th, char *data, size_t optlen)
 {
 	uint32_t prev_ack_to_tap = conn->seq_ack_to_tap;
 	uint32_t prev_wnd_to_tap = conn->wnd_to_tap;
@@ -1629,77 +1298,13 @@ static int do_tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags,
 	return 1;
 }
 
-static int tcp_buf_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
-{
-	size_t optlen = 0;
-	struct iovec *iov;
-	size_t ip_len;
-	int ret;
-
-	/* Options: MSS, NOP and window scale (8 bytes) */
-	if (flags & SYN)
-		optlen = OPT_MSS_LEN + 1 + OPT_WS_LEN;
-
-	if (CONN_V4(conn)) {
-		struct tcp4_l2_flags_buf_t *b4;
-
-		iov = tcp4_l2_flags_iov + tcp4_l2_flags_buf_used;
-		b4 = tcp4_l2_flags_buf + tcp4_l2_flags_buf_used++;
-
-		ret = do_tcp_send_flag(c, conn, flags, &b4->th, b4->opts,
-				       optlen);
-		if (ret <= 0)
-			return ret;
-
-		ip_len = ipv4_fill_headers(c, conn, &b4->iph, optlen,
-					   NULL, conn->seq_to_tap);
-
-		iov->iov_len = tap_iov_len(c, &b4->taph, ip_len);
-
-		if (flags & DUP_ACK) {
-
-			memcpy(b4 + 1, b4, sizeof(*b4));
-			(iov + 1)->iov_len = iov->iov_len;
-			tcp4_l2_flags_buf_used++;
-		}
-
-		if (tcp4_l2_flags_buf_used > ARRAY_SIZE(tcp4_l2_flags_buf) - 2)
-			tcp_buf_l2_flags_flush(c);
-	} else {
-		struct tcp6_l2_flags_buf_t *b6;
-
-		iov = tcp6_l2_flags_iov + tcp6_l2_flags_buf_used;
-		b6 = tcp6_l2_flags_buf + tcp6_l2_flags_buf_used++;
-
-		ret = do_tcp_send_flag(c, conn, flags, &b6->th, b6->opts,
-				       optlen);
-		if (ret <= 0)
-			return ret;
-
-		ip_len = ipv6_fill_headers(c, conn, &b6->ip6h, optlen,
-					   conn->seq_to_tap);
-
-		iov->iov_len = tap_iov_len(c, &b6->taph, ip_len);
-
-		if (flags & DUP_ACK) {
-			memcpy(b6 + 1, b6, sizeof(*b6));
-			(iov + 1)->iov_len = iov->iov_len;
-			tcp6_l2_flags_buf_used++;
-		}
-
-		if (tcp6_l2_flags_buf_used > ARRAY_SIZE(tcp6_l2_flags_buf) - 2)
-			tcp_buf_l2_flags_flush(c);
-	}
-
-	return 0;
-}
 
 /**
  * tcp_rst_do() - Reset a tap connection: send RST segment to tap, close socket
  * @c:		Execution context
  * @conn:	Connection pointer
  */
-static void tcp_rst_do(struct ctx *c, struct tcp_tap_conn *conn)
+void tcp_rst_do(struct ctx *c, struct tcp_tap_conn *conn)
 {
 	if (conn->events == CLOSED)
 		return;
@@ -1813,14 +1418,6 @@ int tcp_conn_new_sock(const struct ctx *c, sa_family_t af)
 	return s;
 }
 
-static uint16_t tcp_buf_conn_tap_mss(const struct tcp_tap_conn *conn)
-{
-	if (CONN_V4(conn))
-		return MSS4;
-
-	return MSS6;
-}
-
 /**
  * tcp_conn_tap_mss() - Get MSS value advertised by tap/guest
  * @conn:	Connection pointer
@@ -2072,179 +1669,6 @@ static int tcp_sock_consume(const struct tcp_tap_conn *conn, uint32_t ack_seq)
 	return 0;
 }
 
-/**
- * tcp_data_to_tap() - Finalise (queue) highest-numbered scatter-gather buffer
- * @c:		Execution context
- * @conn:	Connection pointer
- * @plen:	Payload length at L4
- * @no_csum:	Don't compute IPv4 checksum, use the one from previous buffer
- * @seq:	Sequence number to be sent
- */
-static void tcp_data_to_tap(const struct ctx *c, struct tcp_tap_conn *conn,
-			    ssize_t plen, int no_csum, uint32_t seq)
-{
-	uint32_t *seq_update = &conn->seq_to_tap;
-	struct iovec *iov;
-	size_t ip_len;
-
-	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;
-
-		tcp4_l2_buf_seq_update[tcp4_l2_buf_used].seq = seq_update;
-		tcp4_l2_buf_seq_update[tcp4_l2_buf_used].len = plen;
-
-		ip_len = ipv4_fill_headers(c, conn, &b->iph, plen,
-					   check, seq);
-
-		iov = tcp4_l2_iov + tcp4_l2_buf_used++;
-		iov->iov_len = tap_iov_len(c, &b->taph, ip_len);
-		if (tcp4_l2_buf_used > ARRAY_SIZE(tcp4_l2_buf) - 1)
-			tcp_buf_l2_data_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;
-
-		ip_len = ipv6_fill_headers(c, conn, &b->ip6h, plen, seq);
-
-		iov = tcp6_l2_iov + tcp6_l2_buf_used++;
-		iov->iov_len = tap_iov_len(c, &b->taph, ip_len);
-		if (tcp6_l2_buf_used > ARRAY_SIZE(tcp6_l2_buf) - 1)
-			tcp_buf_l2_data_flush(c);
-	}
-}
-
-/**
- * tcp_buf_data_from_sock() - Handle new data from socket, queue to tap, in window
- * @c:		Execution context
- * @conn:	Connection pointer
- *
- * Return: negative on connection reset, 0 otherwise
- *
- * #syscalls recvmsg
- */
-static int tcp_buf_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, plen, v4 = CONN_V4(conn);
-	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;
-
-	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;
-	}
-
-	if (!wnd_scaled || already_sent >= wnd_scaled) {
-		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);
-	if (fill_bufs > TCP_FRAMES) {
-		fill_bufs = TCP_FRAMES;
-		iov_rem = 0;
-	} else {
-		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;
-
-	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))) {
-		tcp_buf_l2_data_flush(c);
-
-		/* Silence Coverity CWE-125 false positive */
-		tcp4_l2_buf_used = tcp6_l2_buf_used = 0;
-	}
-
-	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;
-		else
-			iov->iov_base = &tcp6_l2_buf[tcp6_l2_buf_used + i].data;
-		iov->iov_len = mss;
-	}
-	if (iov_rem)
-		iov_sock[fill_bufs].iov_len = iov_rem;
-
-	/* Receive into buffers, don't dequeue until acknowledged by guest. */
-	do
-		len = recvmsg(s, &mh_sock, MSG_PEEK);
-	while (len < 0 && errno == EINTR);
-
-	if (len < 0)
-		goto err;
-
-	if (!len) {
-		if ((conn->events & (SOCK_FIN_RCVD | TAP_FIN_SENT)) == SOCK_FIN_RCVD) {
-			if ((ret = tcp_buf_send_flag(c, conn, FIN | ACK))) {
-				tcp_rst(c, conn);
-				return ret;
-			}
-
-			conn_event(c, conn, TAP_FIN_SENT);
-		}
-
-		return 0;
-	}
-
-	sendlen = len - already_sent;
-	if (sendlen <= 0) {
-		conn_flag(c, conn, STALLED);
-		return 0;
-	}
-
-	conn_flag(c, conn, ~STALLED);
-
-	send_bufs = DIV_ROUND_UP(sendlen, mss);
-	last_len = sendlen - (send_bufs - 1) * mss;
-
-	/* Likely, some new data was acked too. */
-	tcp_update_seqack_wnd(c, conn, 0, NULL);
-
-	/* Finally, queue to tap */
-	plen = mss;
-	seq = conn->seq_to_tap;
-	for (i = 0; i < send_bufs; i++) {
-		int no_csum = i && i != send_bufs - 1 && tcp4_l2_buf_used;
-
-		if (i == send_bufs - 1)
-			plen = last_len;
-
-		tcp_data_to_tap(c, conn, plen, no_csum, seq);
-		seq += plen;
-	}
-
-	conn_flag(c, conn, ACK_FROM_TAP_DUE);
-
-	return 0;
-
-err:
-	if (errno != EAGAIN && errno != EWOULDBLOCK) {
-		ret = -errno;
-		tcp_rst(c, conn);
-	}
-
-	return ret;
-}
 
 /**
  * tcp_data_from_tap() - tap/guest data for established connection
diff --git a/tcp_buf.c b/tcp_buf.c
new file mode 100644
index 000000000000..d70e7f810e4a
--- /dev/null
+++ b/tcp_buf.c
@@ -0,0 +1,569 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+/* PASST - Plug A Simple Socket Transport
+ *  for qemu/UNIX domain socket mode
+ *
+ * PASTA - Pack A Subtle Tap Abstraction
+ *  for network namespace/tap device mode
+ *
+ * tcp_buf.c - TCP L2-L4 translation state machine
+ *
+ * Copyright (c) 2020-2022 Red Hat GmbH
+ * Author: Stefano Brivio <sbrivio@redhat.com>
+ */
+
+#include <stddef.h>
+#include <stdint.h>
+#include <limits.h>
+#include <string.h>
+#include <errno.h>
+
+#include <netinet/ip.h>
+
+#include <linux/tcp.h>
+
+#include "util.h"
+#include "ip.h"
+#include "passt.h"
+#include "tap.h"
+#include "siphash.h"
+#include "inany.h"
+#include "tcp_conn.h"
+#include "tcp_internal.h"
+#include "tcp_buf.h"
+
+#define TCP_FRAMES_MEM			128
+#define TCP_FRAMES							\
+	(c->mode == MODE_PASST ? TCP_FRAMES_MEM : 1)
+
+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
+
+#define MSS4	ROUND_DOWN(USHRT_MAX - sizeof(struct tcp4_l2_head), 4)
+#define MSS6	ROUND_DOWN(USHRT_MAX - sizeof(struct tcp6_l2_head), 4)
+
+/**
+ * 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 */
+
+/**
+ * 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
+ */
+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 */
+#ifdef __AVX2__
+} __attribute__ ((packed, aligned(32)))
+#else
+} __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];
+
+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];
+
+static struct tcp_buf_seq_update tcp6_l2_buf_seq_update[TCP_FRAMES_MEM];
+
+static unsigned int tcp6_l2_buf_used;
+
+/* recvmsg()/sendmsg() data for tap */
+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];
+
+/**
+ * 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;
+
+/**
+ * tcp_buf_update_l2() - Update L2 buffers with Ethernet and IPv4 addresses
+ * @eth_d:	Ethernet destination address, NULL if unchanged
+ * @eth_s:	Ethernet source address, NULL if unchanged
+ */
+void tcp_buf_update_l2(const unsigned char *eth_d, const unsigned char *eth_s)
+{
+	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];
+
+		tap_update_mac(&b4->taph, eth_d, eth_s);
+		tap_update_mac(&b6->taph, eth_d, eth_s);
+		tap_update_mac(&b4f->taph, eth_d, eth_s);
+		tap_update_mac(&b6f->taph, eth_d, eth_s);
+	}
+}
+
+/**
+ * tcp_buf_sock4_iov_init() - Initialise scatter-gather L2 buffers for IPv4 sockets
+ * @c:		Execution context
+ */
+void tcp_buf_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 }
+		};
+	}
+
+	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)
+		};
+	}
+
+	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);
+
+	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);
+}
+
+/**
+ * tcp_buf_sock6_iov_init() - Initialise scatter-gather L2 buffers for IPv6 sockets
+ * @c:		Execution context
+ */
+void tcp_buf_sock6_iov_init(const struct ctx *c)
+{
+	struct iovec *iov;
+	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 }
+		};
+	}
+
+	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)
+		};
+	}
+
+	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);
+
+	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);
+}
+
+/**
+ * tcp_buf_l2_flags_flush() - Send out buffers for segments with no data (flags)
+ * @c:		Execution context
+ */
+void tcp_buf_l2_flags_flush(const struct ctx *c)
+{
+	tap_send_frames(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);
+	tcp4_l2_flags_buf_used = 0;
+}
+
+/**
+ * tcp_buf_l2_data_flush() - Send out buffers for segments with data
+ * @c:		Execution context
+ */
+void tcp_buf_l2_data_flush(const struct ctx *c)
+{
+	unsigned i;
+	size_t m;
+
+	m = tap_send_frames(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);
+	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;
+}
+
+int tcp_buf_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
+{
+	size_t optlen = 0;
+	struct iovec *iov;
+	size_t ip_len;
+	int ret;
+
+	/* Options: MSS, NOP and window scale (8 bytes) */
+	if (flags & SYN)
+		optlen = OPT_MSS_LEN + 1 + OPT_WS_LEN;
+
+	if (CONN_V4(conn)) {
+		struct tcp4_l2_flags_buf_t *b4;
+
+		iov = tcp4_l2_flags_iov + tcp4_l2_flags_buf_used;
+		b4 = tcp4_l2_flags_buf + tcp4_l2_flags_buf_used++;
+
+		ret = do_tcp_send_flag(c, conn, flags, &b4->th, b4->opts,
+				       optlen);
+		if (ret <= 0)
+			return ret;
+
+		ip_len = ipv4_fill_headers(c, conn, &b4->iph, optlen,
+					   NULL, conn->seq_to_tap);
+
+		iov->iov_len = tap_iov_len(c, &b4->taph, ip_len);
+
+		if (flags & DUP_ACK) {
+
+			memcpy(b4 + 1, b4, sizeof(*b4));
+			(iov + 1)->iov_len = iov->iov_len;
+			tcp4_l2_flags_buf_used++;
+		}
+
+		if (tcp4_l2_flags_buf_used > ARRAY_SIZE(tcp4_l2_flags_buf) - 2)
+			tcp_buf_l2_flags_flush(c);
+	} else {
+		struct tcp6_l2_flags_buf_t *b6;
+
+		iov = tcp6_l2_flags_iov + tcp6_l2_flags_buf_used;
+		b6 = tcp6_l2_flags_buf + tcp6_l2_flags_buf_used++;
+
+		ret = do_tcp_send_flag(c, conn, flags, &b6->th, b6->opts,
+				       optlen);
+		if (ret <= 0)
+			return ret;
+
+		ip_len = ipv6_fill_headers(c, conn, &b6->ip6h, optlen,
+					   conn->seq_to_tap);
+
+		iov->iov_len = tap_iov_len(c, &b6->taph, ip_len);
+
+		if (flags & DUP_ACK) {
+			memcpy(b6 + 1, b6, sizeof(*b6));
+			(iov + 1)->iov_len = iov->iov_len;
+			tcp6_l2_flags_buf_used++;
+		}
+
+		if (tcp6_l2_flags_buf_used > ARRAY_SIZE(tcp6_l2_flags_buf) - 2)
+			tcp_buf_l2_flags_flush(c);
+	}
+
+	return 0;
+}
+
+uint16_t tcp_buf_conn_tap_mss(const struct tcp_tap_conn *conn)
+{
+	if (CONN_V4(conn))
+		return MSS4;
+
+	return MSS6;
+}
+
+/**
+ * tcp_data_to_tap() - Finalise (queue) highest-numbered scatter-gather buffer
+ * @c:		Execution context
+ * @conn:	Connection pointer
+ * @plen:	Payload length at L4
+ * @no_csum:	Don't compute IPv4 checksum, use the one from previous buffer
+ * @seq:	Sequence number to be sent
+ */
+static void tcp_data_to_tap(const struct ctx *c, struct tcp_tap_conn *conn,
+			    ssize_t plen, int no_csum, uint32_t seq)
+{
+	uint32_t *seq_update = &conn->seq_to_tap;
+	struct iovec *iov;
+	size_t ip_len;
+
+	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;
+
+		tcp4_l2_buf_seq_update[tcp4_l2_buf_used].seq = seq_update;
+		tcp4_l2_buf_seq_update[tcp4_l2_buf_used].len = plen;
+
+		ip_len = ipv4_fill_headers(c, conn, &b->iph, plen,
+					   check, seq);
+
+		iov = tcp4_l2_iov + tcp4_l2_buf_used++;
+		iov->iov_len = tap_iov_len(c, &b->taph, ip_len);
+		if (tcp4_l2_buf_used > ARRAY_SIZE(tcp4_l2_buf) - 1)
+			tcp_buf_l2_data_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;
+
+		ip_len = ipv6_fill_headers(c, conn, &b->ip6h, plen, seq);
+
+		iov = tcp6_l2_iov + tcp6_l2_buf_used++;
+		iov->iov_len = tap_iov_len(c, &b->taph, ip_len);
+		if (tcp6_l2_buf_used > ARRAY_SIZE(tcp6_l2_buf) - 1)
+			tcp_buf_l2_data_flush(c);
+	}
+}
+
+/**
+ * tcp_buf_data_from_sock() - Handle new data from socket, queue to tap, in window
+ * @c:		Execution context
+ * @conn:	Connection pointer
+ *
+ * Return: negative on connection reset, 0 otherwise
+ *
+ * #syscalls recvmsg
+ */
+int tcp_buf_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, plen, v4 = CONN_V4(conn);
+	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;
+
+	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;
+	}
+
+	if (!wnd_scaled || already_sent >= wnd_scaled) {
+		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);
+	if (fill_bufs > TCP_FRAMES) {
+		fill_bufs = TCP_FRAMES;
+		iov_rem = 0;
+	} else {
+		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;
+
+	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))) {
+		tcp_buf_l2_data_flush(c);
+
+		/* Silence Coverity CWE-125 false positive */
+		tcp4_l2_buf_used = tcp6_l2_buf_used = 0;
+	}
+
+	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;
+		else
+			iov->iov_base = &tcp6_l2_buf[tcp6_l2_buf_used + i].data;
+		iov->iov_len = mss;
+	}
+	if (iov_rem)
+		iov_sock[fill_bufs].iov_len = iov_rem;
+
+	/* Receive into buffers, don't dequeue until acknowledged by guest. */
+	do
+		len = recvmsg(s, &mh_sock, MSG_PEEK);
+	while (len < 0 && errno == EINTR);
+
+	if (len < 0)
+		goto err;
+
+	if (!len) {
+		if ((conn->events & (SOCK_FIN_RCVD | TAP_FIN_SENT)) == SOCK_FIN_RCVD) {
+			if ((ret = tcp_buf_send_flag(c, conn, FIN | ACK))) {
+				tcp_rst(c, conn);
+				return ret;
+			}
+
+			conn_event(c, conn, TAP_FIN_SENT);
+		}
+
+		return 0;
+	}
+
+	sendlen = len - already_sent;
+	if (sendlen <= 0) {
+		conn_flag(c, conn, STALLED);
+		return 0;
+	}
+
+	conn_flag(c, conn, ~STALLED);
+
+	send_bufs = DIV_ROUND_UP(sendlen, mss);
+	last_len = sendlen - (send_bufs - 1) * mss;
+
+	/* Likely, some new data was acked too. */
+	tcp_update_seqack_wnd(c, conn, 0, NULL);
+
+	/* Finally, queue to tap */
+	plen = mss;
+	seq = conn->seq_to_tap;
+	for (i = 0; i < send_bufs; i++) {
+		int no_csum = i && i != send_bufs - 1 && tcp4_l2_buf_used;
+
+		if (i == send_bufs - 1)
+			plen = last_len;
+
+		tcp_data_to_tap(c, conn, plen, no_csum, seq);
+		seq += plen;
+	}
+
+	conn_flag(c, conn, ACK_FROM_TAP_DUE);
+
+	return 0;
+
+err:
+	if (errno != EAGAIN && errno != EWOULDBLOCK) {
+		ret = -errno;
+		tcp_rst(c, conn);
+	}
+
+	return ret;
+}
diff --git a/tcp_buf.h b/tcp_buf.h
new file mode 100644
index 000000000000..d23031252002
--- /dev/null
+++ b/tcp_buf.h
@@ -0,0 +1,17 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later
+ * Copyright (c) 2021 Red Hat GmbH
+ * Author: Stefano Brivio <sbrivio@redhat.com>
+ */
+
+#ifndef TCP_BUF_H
+#define TCP_BUF_H
+
+void tcp_buf_sock4_iov_init(const struct ctx *c);
+void tcp_buf_sock6_iov_init(const struct ctx *c);
+void tcp_buf_l2_flags_flush(const struct ctx *c);
+void tcp_buf_l2_data_flush(const struct ctx *c);
+uint16_t tcp_buf_conn_tap_mss(const struct tcp_tap_conn *conn);
+int tcp_buf_data_from_sock(struct ctx *c, struct tcp_tap_conn *conn);
+int tcp_buf_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags);
+
+#endif  /*TCP_BUF_H */
diff --git a/tcp_internal.h b/tcp_internal.h
new file mode 100644
index 000000000000..36eb2463dd5a
--- /dev/null
+++ b/tcp_internal.h
@@ -0,0 +1,78 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later
+ * Copyright (c) 2021 Red Hat GmbH
+ * Author: Stefano Brivio <sbrivio@redhat.com>
+ */
+
+#ifndef TCP_INTERNAL_H
+#define TCP_INTERNAL_H
+
+#define MAX_WS				8
+#define MAX_WINDOW			(1 << (16 + (MAX_WS)))
+
+#define SEQ_LE(a, b)			((b) - (a) < MAX_WINDOW)
+#define SEQ_LT(a, b)			((b) - (a) - 1 < MAX_WINDOW)
+#define SEQ_GE(a, b)			((a) - (b) < MAX_WINDOW)
+#define SEQ_GT(a, b)			((a) - (b) - 1 < MAX_WINDOW)
+
+#define FIN		(1 << 0)
+#define SYN		(1 << 1)
+#define RST		(1 << 2)
+#define ACK		(1 << 4)
+
+/* Flags for internal usage */
+#define DUP_ACK		(1 << 5)
+#define OPT_EOL		0
+#define OPT_NOP		1
+#define OPT_MSS		2
+#define OPT_MSS_LEN	4
+#define OPT_WS		3
+#define OPT_WS_LEN	3
+#define OPT_SACKP	4
+#define OPT_SACK	5
+#define OPT_TS		8
+
+#define CONN_V4(conn)		(!!inany_v4(&(conn)->faddr))
+#define CONN_V6(conn)		(!CONN_V4(conn))
+
+extern char tcp_buf_discard [MAX_WINDOW];
+
+void conn_flag_do(const struct ctx *c, struct tcp_tap_conn *conn,
+		  unsigned long flag);
+#define conn_flag(c, conn, flag)					\
+	do {								\
+		flow_trace(conn, "flag at %s:%i", __func__, __LINE__);	\
+		conn_flag_do(c, conn, flag);				\
+	} while (0)
+
+
+void conn_event_do(const struct ctx *c, struct tcp_tap_conn *conn,
+		   unsigned long event);
+#define conn_event(c, conn, event)					\
+	do {								\
+		flow_trace(conn, "event at %s:%i", __func__, __LINE__);	\
+		conn_event_do(c, conn, event);				\
+	} while (0)
+
+void tcp_rst_do(struct ctx *c, struct tcp_tap_conn *conn);
+#define tcp_rst(c, conn)						\
+	do {								\
+		flow_dbg((conn), "TCP reset at %s:%i", __func__, __LINE__); \
+		tcp_rst_do(c, conn);					\
+	} while (0)
+
+
+size_t ipv4_fill_headers(const struct ctx *c,
+			 const struct tcp_tap_conn *conn,
+			 struct iphdr *iph, size_t plen,
+			 const uint16_t *check, uint32_t seq);
+size_t ipv6_fill_headers(const struct ctx *c,
+			 const struct tcp_tap_conn *conn,
+			 struct ipv6hdr *ip6h, size_t plen,
+			 uint32_t seq);
+
+int tcp_update_seqack_wnd(const struct ctx *c, struct tcp_tap_conn *conn,
+			  int force_seq, struct tcp_info *tinfo);
+int do_tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags,
+		     struct tcphdr *th, char *data, size_t optlen);
+
+#endif /* TCP_INTERNAL_H */
-- 
@@ -0,0 +1,78 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later
+ * Copyright (c) 2021 Red Hat GmbH
+ * Author: Stefano Brivio <sbrivio@redhat.com>
+ */
+
+#ifndef TCP_INTERNAL_H
+#define TCP_INTERNAL_H
+
+#define MAX_WS				8
+#define MAX_WINDOW			(1 << (16 + (MAX_WS)))
+
+#define SEQ_LE(a, b)			((b) - (a) < MAX_WINDOW)
+#define SEQ_LT(a, b)			((b) - (a) - 1 < MAX_WINDOW)
+#define SEQ_GE(a, b)			((a) - (b) < MAX_WINDOW)
+#define SEQ_GT(a, b)			((a) - (b) - 1 < MAX_WINDOW)
+
+#define FIN		(1 << 0)
+#define SYN		(1 << 1)
+#define RST		(1 << 2)
+#define ACK		(1 << 4)
+
+/* Flags for internal usage */
+#define DUP_ACK		(1 << 5)
+#define OPT_EOL		0
+#define OPT_NOP		1
+#define OPT_MSS		2
+#define OPT_MSS_LEN	4
+#define OPT_WS		3
+#define OPT_WS_LEN	3
+#define OPT_SACKP	4
+#define OPT_SACK	5
+#define OPT_TS		8
+
+#define CONN_V4(conn)		(!!inany_v4(&(conn)->faddr))
+#define CONN_V6(conn)		(!CONN_V4(conn))
+
+extern char tcp_buf_discard [MAX_WINDOW];
+
+void conn_flag_do(const struct ctx *c, struct tcp_tap_conn *conn,
+		  unsigned long flag);
+#define conn_flag(c, conn, flag)					\
+	do {								\
+		flow_trace(conn, "flag at %s:%i", __func__, __LINE__);	\
+		conn_flag_do(c, conn, flag);				\
+	} while (0)
+
+
+void conn_event_do(const struct ctx *c, struct tcp_tap_conn *conn,
+		   unsigned long event);
+#define conn_event(c, conn, event)					\
+	do {								\
+		flow_trace(conn, "event at %s:%i", __func__, __LINE__);	\
+		conn_event_do(c, conn, event);				\
+	} while (0)
+
+void tcp_rst_do(struct ctx *c, struct tcp_tap_conn *conn);
+#define tcp_rst(c, conn)						\
+	do {								\
+		flow_dbg((conn), "TCP reset at %s:%i", __func__, __LINE__); \
+		tcp_rst_do(c, conn);					\
+	} while (0)
+
+
+size_t ipv4_fill_headers(const struct ctx *c,
+			 const struct tcp_tap_conn *conn,
+			 struct iphdr *iph, size_t plen,
+			 const uint16_t *check, uint32_t seq);
+size_t ipv6_fill_headers(const struct ctx *c,
+			 const struct tcp_tap_conn *conn,
+			 struct ipv6hdr *ip6h, size_t plen,
+			 uint32_t seq);
+
+int tcp_update_seqack_wnd(const struct ctx *c, struct tcp_tap_conn *conn,
+			  int force_seq, struct tcp_info *tinfo);
+int do_tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags,
+		     struct tcphdr *th, char *data, size_t optlen);
+
+#endif /* TCP_INTERNAL_H */
-- 
2.42.0