FreeBSD xn ether issues as DOMU under NetBSD DOM0

[smj%ma.sdf.org@localhost]

Miguel Clara reports on freebsd-xen an issue with netfront/netback for 
FreeBSD DOMUs under NetBSD DOM0.   I've confirmed this behaviour under 
NetBSD 7.x XEN4.5.  I've tried disabling features both in the FreeBSD kernel
and using ifconfig/sysctl but have only been able to achieve differing 
states of "bad" networking.  

(from the freebsd-xen mailing list):

See this PR from 2014 --> https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=188369

(there another earlier about the same error message on 9.X and that was fixed on 10-CURRENT at the time, but was "broke" later)

The issue happen with any NetBSD version from 5.x to 7.x with any xen version but it only stared in FreeBSD 10-Current, so I was always inclined to this being a regression in 10 not a NetBSD backend issue. Also I did post in netbsd mailling list and the reply was something in the lines of "was working fine, nothing changed at our side, so its probably FreeBSD problem"

Also as I posted in that PR I experienced a similar issue with Windows GPLPV drivers and the dev fixed it, the issue was that certain features were not supported and the frontend just assume they were cause that's how it works on linux, and I'm no expert so I can go into much detail, but it was related to checksum, tso and I think gso.

I tried to bisect the git commits, but there was simply to much changes, what I did find was that this never worked on 10.x +++ only 9, anyway everything was reported there and eventually I got no replies back so I gave up on NetBSD+FreeBSD-10.

Unfortunately this needs someone with proper skills on either side (netbsd/freeBSD) to be fixed, the perfect solution would be to add NetBSD backend support for this things (just not sure if its worth the time for the possible performance gain!? - also that's a question for the NetBSD folks ofc)