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

[Scott Damron <sdamron@xxxxxxxxx>]

There are some things you should use dedicated machines for, a gateway
is one of those things in my opinion.  Why would you run a Xen kernel
on your gateway to the internet?  Now, I would run a base hypervisor
and build my gateway inside of it as a guest, but not vise versa.  My
two centavos.

Scott

On Thu, Sep 8, 2011 at 11:05 PM, Chris Angelico <rosuav@xxxxxxxxx> wrote:
> 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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