freebsd domu on netbsd dom0 kernel panic

[Gunter Wambaugh]

I've been able to get a FreeBSD 8 Xen PV domU running successfully on
RedHat 5; however, when I try to run the same image under NetBSD Xen,
I get a kernel panic. If I comment out the 'vif' line it boots, but I
have no network obviously. I've got a RedHat domU that I moved as
well, and it works perfectly under NetBSD.

The guest:
uname -a
FreeBSD fbworkstation 8.0-RELEASE-p2 FreeBSD 8.0-RELEASE-p2 #0: Fri
Apr  9 18:57:44 UTC 2010     root at freebsdxen:/usr/obj/usr/src/sys/XEN
i386

The host:
uname -a
NetBSD vdesktop 5.0.2 NetBSD 5.0.2 (XEN3_DOM0) #0: Sat Feb  6 13:44:13
UTC 2010  builds at b8.netbsd.org:/home/builds/ab/netbsd-5-0-2-RELEASE/amd64/201002061851Z-obj/home/builds/ab/netbsd-5-0-2-RELEASE/src/sys/arch/amd64/compile/XEN3_DOM0
amd64

The console message:
xn0: <Virtual Network Interface> at device/vif/0 on xenbus0
xn0: bpf attached
xn0: Ethernet address: 00:16:3e:68:d8:55
WARNING: WITNESS option enabled, expect reduced performance.
panic: mutex ??????/?5? not owned at /usr/src/sys/vm/vm_page.c:425
cpuid = 0
KDB: enter: panic
[thread pid 12 tid 100023 ]
Stopped at      kdb_enter+0x3a: movl    $0,kdb_why
db>

I'd be happy to provide any further details, and even step through the
debugger if someone could tell me what to look for.

Thanks!