Re: [Xen-users] New to Xen: safety concerns (Linux Dom0, Windows DomU)

["Drake Wilson" <drake@xxxxxxxxxxxxxx>]

On Friday, September 09, 2011 1:21 PM, "Chris Angelico" <rosuav@xxxxxxxxx> 
wrote:
> Hi! I'm looking at installing Xen to play around with Windows as well
> as Linux, but since it's a somewhat invasive install, figured it'd be
> safer to ask some questions first.
> 
> I have a Linux box that's running Ubuntu 10.10 happily as a desktop
> and router. What I'm trying to do is run Windows on it in addition,
> and have Windows have direct access to the nVidia card - which is what
> most virtualizers won't do. But above all, I want to keep the Linux
> half running. Whatever happens, this is to remain functional as the
> router. If Windows has a problem, I need to be able to restart it
> without bringing Linux down.

One of your problems here is that VGA passthrough (at least if you want
it to hit the domU's BIOS) can actually be very hit-or-miss, though it's
gotten much better over time:

  http://wiki.xensource.com/xenwiki/XenVGAPassthrough

The secondary problem with this is that if the Windows half crashes, and
you need to restart it, how are you going to access the Xen tools to
restart it if Windows took the display adapter with it?  These things are
often solvable, but it is worth thinking about it in advance (per below).

> Also, should Xen not work out, I'd like to be able to uninstall it and
> return to the clean Linux that's there now. Googling for information
> on this last point has turned up a number of people who've had no
> trouble, but also a number who did, and I'm not sure if the issues are
> self-inflicted, nor if they're only in older versions of Xen.

Can you describe which trouble you're worried about in particular, if any?

Newer Linux kernels have Xen and non-Xen boot processes that are closer to
each other (I'm thinking 3.0.0 particularly; I don't know what Ubuntu 10.10
has), and with things like UUID-based filesystem detection (which Ubuntu has
done as standard for a while, but not necessarily if you started from a much
older version) the differences in exposed hardware can often be automatically
dealt with.  Older Linuxes had specialized Xen versions of the kernel, and
so you'd have to change boot configurations around more.  In either case it'd be
advisable to have a rescue disk handy just in case.  But generally speaking
switching a Linux system between dom0 and raw is a very reversible operation
unless/until you configure it to depend strongly on Xen-specific or very 
low-level
hardware operations.

> 2) Can a DomU Windows have full access to the hardware?

You should think about what you mean by "full access".  You may be able to pass
through most of the interface PCI devices and such (with work), but if you will
still need access to the Linux half then you must arrange for enough console or
network devices to be routed to it for that purpose.

Note also that you must have a hardware IOMMU for PCI passthrough to HVM guests,
according to http://wiki.xensource.com/xenwiki/XenPCIpassthrough, and I don't
believe Windows can be run paravirtualized since the kernel hasn't been ported
(for obvious reasons).  In practice this may mean some fairly high-class 
hardware,
depending on your configuration.

FYI, I'm currently gradually in the process of setting up a split server/desktop
configuration as well involving splitting up the PCI devices, though both of the
larger domains will be domU (with a small dom0) and it's Debian GNU/Linux all 
the
way through (no Windows---yet; I might wind up spinning up a Windows domU as 
well
but I wouldn't be giving it the GPU or anything like that).

Good luck!

   ---> Drake Wilson

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