TSO failure from FreeBSD HVM PV netfront guest to Dom0

["Justin T. Gibbs" <gibbs@scsiguy.com>]

Configuration
=============
Dom0: OpenSuSE 11.2 with their Xen 4.0.0 package.  I believe
       this to be approximately 4.0.0-RC1

DomU: FreeBSD 9-current x86_64 HVM, PV netfront and blkfront

With TSO enabled, network connections with external machines
and other guests (OpenSuSE 11.2 x86_64 PV) operate as expected.
However, TX requests larger than the negotiated MSS appear to be
dropped when Dom0 is the end point.  You can see the behavior in
this tcpdump log here: http://www.pastebin.org/84142.  It was
captured from the FreeBSD DomU (50.50.50.85).  bashful is the
Dom0.

I am still reviewing the FreeBSD netfront driver to see if
there is something wrong it is doing, but since tcpdump confirms
TSO to be functioning correctly (TX requests larger than MSS get
through fine) to everyone but Dom0, my hunch is that the problem
is somewhere in Dom0.  Any suggestions on where to look?

Thanks,
Justin