Date: Sat, 13 Sep 2014 16:54:08 -0400 (EDT) From: Rick Macklem <rmacklem@uoguelph.ca> To: Hans Petter Selasky <hps@selasky.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org, Adrian Chadd <adrian@freebsd.org> Subject: Re: svn commit: r271504 - in head/sys: dev/oce dev/vmware/vmxnet3 dev/xen/netfront net netinet ofed/drivers/net/mlx4 Message-ID: <1935263641.35880661.1410641648947.JavaMail.root@uoguelph.ca> In-Reply-To: <5414AB58.6040400@selasky.org>
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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 > >
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