svn commit: r271504 - in head/sys: dev/oce dev/vmware/vmxnet3 dev/xen/netfront net netinet ofed/drivers/net/mlx4

[Rick Macklem]

Hans Petter Selasky wrote:
> On 09/13/14 22:35, Hans Petter Selasky wrote:
> >
> > I see the default set to (65536 - 4). I don't know why you
> > subtracted 4
> > but I would have expected the max ethernet header length to be
> > subtracted
> > here?
> 
> You mean to say that the default should be
> 
> 65535 - ethernet header - vlan header ?
> 
Almost. It is actually:
65536 - ethernet header - vlan header
(or the min of IP_MAXPACKET vs 32*MCLBYTES - ethernet header - vlan header
 if you want to cover your butt for the case where the value of
 MCLBYTES is changed) IP_MAXPACKET (65535) comes from the fact that
some devices use the iplen field of the ip header in the TSO segment
for its length, I think? (Some do not and can support TSO segments
greater than IP_MAXPACKET in length, but again, the default shouldn't
assume this nor should it assume the device does the vlan header in
hardware --> "- vlan header" to be safe for default.)

This is because there are lots of broken drivers (basically any one
that has a limit of 32 transmit segments) and this at least makes
them work correctly. Unfortunately they still do a lot of m_defrag()
calls for this case, but with a patch like yours, the drivers may
eventually get patched to use the max_frags setting and then avoid
the need to do m_defrag() calls.

rick

> --HPS
> 
>