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

[Chris Angelico <rosuav@xxxxxxxxx>]

On Fri, Sep 9, 2011 at 1:41 PM, Drake Wilson <drake@xxxxxxxxxxxxxx> wrote:
> 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

Thanks, an excellent document.

"Xen VGA graphics passthru is a special form of PCI passthru, and PCI
passthru dedicates the PCI device (graphics card) to exactly one
single VM."

I assume I can switch it to a different VM on the fly? That is, boot
with the graphics card dedicated to dom0 Linux, then fire up domU
Windows and hand control over.

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

Googling for 'xen uninstall' shows up a variety of people asking
similar questions:
http://forums.opensuse.org/english/get-technical-help-here/applications/418253-how-uninstall-xen.html
- seems to have uninstalled cleanly
http://www.linuxformat.com/forums/viewtopic.php?t=377 - not too clear
on the question itself there
http://ubuntuforums.org/archive/index.php/t-953793.html - required
some manual cleanup and not sure if it really cleaned up (dated 2008)
http://www.firewall.cx/ftopict-6304.html - no resolution, no responses
at all (dated 2009)

Not enough weight of evidence to turn me away from Xen, but enough to
be concerned about.

> 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),

According to `uname -a` it's currently 2.6.35-30-generic. I could
upgrade the kernel to version 3 I guess, but I'm not a kernel expert
so I'd be navigating unfamiliar waters. It'd be a separate "can I undo
this if things go wrong" question all of its own.

> 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.

This particular box is quite new; 10.10 (I don't like 11.04) was
installed fresh on a bare HD. It may have had a kernel upgrade or two
but nothing particularly earth-shattering.

> 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.

Thank you. I believe you, for I am sure you would not practice on my
inexperience. I wish to do the right thing, and if - I say if - it
really is that easy to reverse, the complexity shall be no obstacle to
our union. Or something like that. (Pirates of Penzance, if you're not
an opera buff.)

>> 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.

I want to play graphical Windows games. It's a 64-bit system with 8GB
of RAM and a fairly new nVidia chipset video card (don't remember the
spec atm), so in theory I should be able to give 2-3GB to a 32-bit
WinXP and let that run happily, while leaving 5-6GB of real RAM for
everything else.

> 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.

Hmm. Is there an easy way to check? It's a high-end Intel motherboard,
and a high-end modern CPU, although I don't have the precise
identifiers to hand.

I'm not afraid of a bit of complexity, but my areas of expertise are
user-level (ring 3) software and networking, not kernels and
hypervisors. Much appreciate your help!

Chris Angelico

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