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)

2 files changed, 18 insertions, 6 deletions

diff --git a/sys/net/if.c b/sys/net/if.c
index dc5681e..3fd6efb 100644
--- a/sys/net/if.c
+++ b/sys/net/if.c
@@ -74,18 +74,18 @@
 #include <net/vnet.h>
 
 #if defined(INET) || defined(INET6)
-/*XXX*/
 #include <netinet/in.h>
 #include <netinet/in_var.h>
+#include <netinet/ip.h>
 #include <netinet/ip_carp.h>
+#ifdef INET
+#include <netinet/if_ether.h>
+#endif /* INET */
 #ifdef INET6
 #include <netinet6/in6_var.h>
 #include <netinet6/in6_ifattach.h>
-#endif
-#endif
-#ifdef INET
-#include <netinet/if_ether.h>
-#endif
+#endif /* INET6 */
+#endif /* INET || INET6 */
 
 #include <security/mac/mac_framework.h>
 
@@ -653,6 +653,13 @@ if_attach_internal(struct ifnet *ifp, int vmove)
 		TAILQ_INSERT_HEAD(&ifp->if_addrhead, ifa, ifa_link);
 		/* Reliably crash if used uninitialized. */
 		ifp->if_broadcastaddr = NULL;
+
+		/* Initialize to max value. */
+		if (ifp->if_hw_tsomax == 0)
+			ifp->if_hw_tsomax = IP_MAXPACKET;
+		KASSERT(ifp->if_hw_tsomax <= IP_MAXPACKET &&
+		    ifp->if_hw_tsomax >= IP_MAXPACKET / 8,
+		    ("%s: tsomax outside of range", __func__));
 	}
 #ifdef VIMAGE
 	else {
diff --git a/sys/net/if_var.h b/sys/net/if_var.h
index ce8f06a..3babc22 100644
--- a/sys/net/if_var.h
+++ b/sys/net/if_var.h
@@ -204,6 +204,11 @@ struct ifnet {
 	u_int	if_fib;			/* interface FIB */
 	u_char	if_alloctype;		/* if_type at time of allocation */
 
+	u_int	if_hw_tsomax;		/* tso burst length limit, the minmum
+					 * is (IP_MAXPACKET / 8).
+					 * XXXAO: Have to find a better place
+					 * for it eventually. */
+
 	/*
 	 * Spare fields are added so that we can modify sensitive data
 	 * structures without changing the kernel binary interface, and must