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RE: [Xen-users] xen and different ARCH

> hey gang, just an odd question. I do a lot of cross compiling of 
> software, all for x86, and got to thinking. It would be a LOT 
> easier to 
> setup a VM that thinks it's an i(3|4|5|)86 instead of 
> building separate 
> tool chains. Does anyone know if I can make a xen machine think it is 
> something other then what the host is, within x86 that is.

Hi Jason,

Kind of :)

I'm the sys admin for a software development team of around 70 or so. We
have a series of server(s) that the guys build software on directly. Its all
Linux (FC) based Java, C & C++ stuff.

Recently, we've been getting new x86_64 boxes in the door and I had the same
?problem/idea?. My experiences so far...

Xen won't allow you to run say a 64bit DomO and have a 32bit DomU beneath
it. They all have to be 64 or 32bit. Don't know the true intricacies of why,
(IIRC to do with how 64 and 32bit machines access memory) but you can't.
Haven't tested VMware properly regarding this yet, but due to its different
approach to virtualisation, it may be possible (if so I could be doing some
serious migration work soon :).

Subsequently, no, you can't have a DomU think its something different
(arch-wise). The kernels (DomO & DomU) have to be of the same arch so if the
DomO is 64bit, your DomU has to be too. This is a problem for us when
building apps (all of ours are for embedded 32bit targets).

To get around these issues, I presently...

# Generally run 64bit. Most of my DomO's are x64_64 FC4/5 boxes. 
# Use a SSH startup hack (introduce setarch into the initscript) so that all
clients that connect get a 32bit session. This works surprisingly well and
stops nasty uname issues (their session always (presuming access via SSH)
thinks its 32bit).
# Create a 32bit template DomU. I have a series of *clean* DomU templates
(created using yum into a chroot on a 32bit machine), that I use when
creating a new 32bit DomU. I just copy the lot across to a new LVM based
VBD, copy across the appropriate 64bit kernel and modules from the DomO (I
keep kernels and modules for both on my DomO just in case), change the usual
settings and light it up. Add in the SSH hack and you have to all intents
and purposes, a 32bit DomU beneath a 64bit DomO.

That's about it. Its kind of messy, but it works. It would be nice to just
use any architecture, but...

(Wonder if VT based Xen Doms can be mixed and matched... Hmmmm....)



Dan Hawker
Linux System Administrator


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