Ubuntu 16.04 (xenial) partial success with NetBSD and Xen

["Aaron J. Grier" <agrier%poofygoof.com@localhost>]

with HVM, I can successfully boot Ubuntu 16.04 (both install and update
from 14.04) under NetBSD-7 Dom0 with both Xen 4.2.3 and 4.6.3.

the Linux kernel option xen_emul_unplug=unnecessary (or =never) is what
gets things nominally working.  this yields a booting system with
emulated disk and PV network, with hypercalls used for Xen services
(clocks, timers, etc).  not quite full PVHVM, but not completely HVM
either.

part of the problem seems to be the "unplug" protocol for emulated
devices which was implemented eight years ago [1] in QEMU, and doesn't
seem to be fully supported in the version shipped with NetBSD's Xen
packages.  (both xentools42 and xentools46 use QEMU 0.10.2.)  it's
supposed to unplug the emulated devices, leaving only the native Xen
block devices.

on the PV front, between Xen 4.2.3 and 4.6.3, pygrub stopped working,
possibly due to libxl's changing the handling of POLLIN and POLLHUP [2].
I didn't pursue tracking this down further, since getting pvgrub seemed
like a better solution.  (broken NFS client for NetBSD-7 slows progress,
too.)

PV Linux kernels boot (with pvgrub), but the ramdisk can't find the Xen
block root device.  updates to blkfront drivers somewhere between kernel
3.13.0 (ubuntu 14.04) and 4.4.0 (ubuntu 16.04) seem to prevent newer
kernels from attaching the Xen block devices.  I haven't tried to boot
anything other than Linux with pvgrub yet.

[1] http://xenbits.xen.org/gitweb/?p=qemu-xen-unstable.git;a=blob;f=i386-dm/README.hvm-pv-magic-ioport-disable;h=656f3c65451a1614185b362e10ae6329666d9511;hb=bc5c93ede039525765a7aa98e1da3386efa1465f

[2] https://lists.xenproject.org/archives/html/xen-devel/2015-04/msg00302.html

-- 
  Aaron J. Grier | "Not your ordinary poofy goof." | agrier%poofygoof.com@localhost