Issue with packet framing on xnb(4) connections and NetBSD domus

[Brian Buhrow]

	hello.  I've noticed that on my Xen server, running FreeBSD-12.2/Xen-4.14.0, I see a lot
of messages like the following from my NetBSD-5 and NetBSD-current domu's: 
xennet0: discarding oversize frame (len=1518)

They happen when network traffic is heavy and when traffic is coming from a physical switch in
the same vlan as the domu's.  Since I don't think packets are  actually oversized on the wire,
the Cisco switch managing this network does not report any giant packets, I think this is a
software problem either on the NetBSD domu's, or the FreeBSD dom0.  Since I've been running
these same NetBSD domu setups on a machine where NetBSD is the dom0, I think this is an issue
with the xnb(4) or netback.c  driver.  Specifically, it looks like it concatinates multiple
packets together before notifying the domu that there is traffic available.  Perhaps that is by
design, since I cannot reproduce the issue on a FreeBSD-domu running on the same FreeBSD-dom0
machine.  In either case, while things are usable, it creates a very noisy log on the
NetBSD-domu machines.  Does the xnb(4) driver forward multiple packets to the domu front ends
and set the length of the message to the sum of the lengths of all the packets it's forwarding,
leaving it to the domu front end driver to separate the packets on reception?  Is there a
parameter that can be set on a per-domu basis to say whether you want
multiple-packets-per-transfer or not from the dom0?

-thanks
-Brian