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Re: [Xen-users] Portability (was Re: Users can provide...)

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Subject: Re: [Xen-users] Portability (was Re: Users can provide...)
From: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Date: Wed, 15 Jun 2005 15:36:16 +0100
Cc: Sascha Retzki <sretzki@xxxxxx>
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> On Wed, Jun 15, 2005 at 02:35:39PM +0100, Mark Williamson wrote:
> > filesystems Linux will support.  *however* this will arguably be most
> > important to people who are a) paranoid about security (highly untrusted
> > guests) or b) use really weird filesystems ;-)
> Well, "weird" may mean anything; Given that the UFS of linux is nicely said
> "dark", and Xen is not a linux-only thing anymore (see *BSD).

True :-)

By "weird filesystems" I really meant that this bootloader approach will 
support any guest filesystem that Linux can support.  This includes the 
commonly used Linux filesystems (ext and ReiserFS), high performance 
filesystems like XFS and Reiser4, and also more unusual things like BeFS.

Adding support to the dom0-based loader shouldn't be *that* hard but does 
require explicit use of a new filesystem library.  Also, I doubt that all 
FSes supported by Linux have such a library available...

I don't know much about Linux's UFS support.  However, if it's able to mount 
UFS read-only that'd be enough.  Can it do this reliably?

> Speaking of it, as there are several Operating Systems out there which
> want to act as a domO-able OS, too - How "portable" do you think Xen is?

Xen itself shouldn't need to be modified at all for this - guests need to be 
ported to use the dom0 and privileged interfaces, which are OS independent...

The major work is in modifying the OS to run on Xen using these interfaces and 
in writing "backend" drivers so that it can support unprivileged domains.

> Furthermore, Not all xentools are written in python, right? One could
> count taht as a bug.

There are a few shell scripts, yes.  Also, the lowlevel code is in C for 
performance and convenience reasons.

Porting an OS to run as dom0 is going to be much harder than fixing the tools 
to work under that OS.  For UNIX-likes, fixing the tools isn't hard, as you 
say (they work on NetBSD as well as Linux, presumably with a few tweaks).  A 
Windows (or Mac OS Xi, for that matter) dom0 is still quite far off.

> I'd love a world where I could boot and run all BSDs, some linuxes, Plan9,
> some other Operating Systems (currently no xen support at all ;) ). So that
> needs a bit more than some shell-script that will just run on unix; Even
> Python may be complicated, but is way better than sticking to #!/bin/sh.

Well, for unpriv guests you'll be able to do that anyhow.  It's just dom0 that 
needs to be running all this stuff.  The issues you've described will 
definitely be very relevant if / when we start running non UNIX-like dom0s 
but I don't think that'll happen for quite a long time.


> > Cheers,
> > Mark
> >
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