Allow drivers to specify a maximum TSO length in bytes if they are

limited in the amount of data they can handle at once.

Drivers can set ifp->if_hw_tsomax before calling ether_ifattach() to
change the limit.

The lowest allowable size is IP_MAXPACKET / 8 (8192 bytes) as anything
less wouldn't be very useful anymore.  The upper limit is still at
IP_MAXPACKET (65536 bytes).  Raising it requires further auditing of
the IPv4/v6 code path's as the length field in the IP header would
overflow leading to confusion in firewalls and others packet handler on
the real size of the packet.

The placement into "struct ifnet" is a bit hackish but the best place
that was found.  When the stack/driver boundary is updated it should
be handled in a better way.

Submitted by:	cperciva (earlier version)
Reviewed by:	cperciva
Tested by:	cperciva
MFC after:	1 week (using spare struct members to preserve ABI)

1 files changed, 14 insertions, 3 deletions

diff --git a/sys/netinet/tcp_var.h b/sys/netinet/tcp_var.h
index eddbd3c..0445d8c 100644
--- a/sys/netinet/tcp_var.h
+++ b/sys/netinet/tcp_var.h
@@ -208,6 +208,8 @@ struct tcpcb {
 	u_int	t_keepintvl;		/* interval between keepalives */
 	u_int	t_keepcnt;		/* number of keepalives before close */
 
+	u_int	t_tsomax;		/* tso burst length limit */
+
 	uint32_t t_ispare[8];		/* 5 UTO, 3 TBD */
 	void	*t_pspare2[4];		/* 4 TBD */
 	uint64_t _pad[6];		/* 6 TBD (1-2 CC/RTT?) */
@@ -324,6 +326,15 @@ struct hc_metrics_lite {	/* must stay in sync with hc_metrics */
 	u_long	rmx_recvpipe;   /* inbound delay-bandwidth product */
 };
 
+/*
+ * Used by tcp_maxmtu() to communicate interface specific features
+ * and limits at the time of connection setup.
+ */
+struct tcp_ifcap {
+	int	ifcap;
+	u_int	tsomax;
+};
+
 #ifndef _NETINET_IN_PCB_H_
 struct in_conninfo;
 #endif /* _NETINET_IN_PCB_H_ */
@@ -782,10 +793,10 @@ void	 tcp_reass_flush(struct tcpcb *);
 void	 tcp_reass_destroy(void);
 #endif
 void	 tcp_input(struct mbuf *, int);
-u_long	 tcp_maxmtu(struct in_conninfo *, int *);
-u_long	 tcp_maxmtu6(struct in_conninfo *, int *);
+u_long	 tcp_maxmtu(struct in_conninfo *, struct tcp_ifcap *);
+u_long	 tcp_maxmtu6(struct in_conninfo *, struct tcp_ifcap *);
 void	 tcp_mss_update(struct tcpcb *, int, int, struct hc_metrics_lite *,
-	    int *);
+	    struct tcp_ifcap *);
 void	 tcp_mss(struct tcpcb *, int);
 int	 tcp_mssopt(struct in_conninfo *);
 struct inpcb *