libvirt users - Jul 2014 - configuring a lxc guest to run 32bits from a 64bits host under libvirt-lxc

Hi there
I have a couple of newbie questions concerning running a 32bits guest in a
64bits host using libvirt - lxc; I’m running libvirt-1.2.5 on fedora18 and
fedora20

(a) is this supported at all, and 
(b) if so how do I configure this for a given guest ?

guessing from the mere existence of
http://www.redhat.com/archives/libvir-list/2011-February/msg01040.html
I would think the answer to (a) is yes; anybody cares to confirm ?

As far as (b), I would expect some flag could be set in the XML description of
my guest so that libvirt can perform the proper syscall to personality() at the
right point in the process hierarchy..
However I have not been able to spot this in the documentation so far, but OTOH
the page on the XML configuration here
http://libvirt.org/format.html
appears to be empty so this might be a temporary hiccup ?

in any case, any hint or insight on this matter would be much appreciated —
Thierry

Hi again

Answering my own e-mail on a few points:

. first off, stupid me, http://libvirt.org/format.html indeed is empty and it
seems to be by design, but the subtrees are not, like e.g.
http://libvirt.org/formatdomain.html

. from this page it seems like using something like
  <os>
    <type arch='i686'>exe</type>
    <init>/sbin/init</init>
  </os>
in the guest XML config should do the trick

I need to investigate this out a little further to see if it does what I am
expecting or not

— Thierry

On 02 Jul 2014, at 00:16, Thierry Parmentelat
<thierry.parmentelat@inria.fr> wrote:
> Hi there
> I have a couple of newbie questions concerning running a 32bits guest in a
64bits host using libvirt - lxc; I’m running libvirt-1.2.5 on fedora18 and
fedora20
> 
> (a) is this supported at all, and 
> (b) if so how do I configure this for a given guest ?
> 
> guessing from the mere existence of
http://www.redhat.com/archives/libvir-list/2011-February/msg01040.html
> I would think the answer to (a) is yes; anybody cares to confirm ?
> 
> As far as (b), I would expect some flag could be set in the XML description
of my guest so that libvirt can perform the proper syscall to personality() at
the right point in the process hierarchy..
> However I have not been able to spot this in the documentation so far, but
OTOH the page on the XML configuration here
> http://libvirt.org/format.html
> appears to be empty so this might be a temporary hiccup ?
> 
> in any case, any hint or insight on this matter would be much appreciated —
Thierry
> 
>

On Wed, Jul 02, 2014 at 12:56:18AM +0200, Thierry Parmentelat
wrote:
> Hi again
> 
> Answering my own e-mail on a few points:
> 
> . first off, stupid me, http://libvirt.org/format.html indeed is empty and
it seems to be by design, but the subtrees are not, like e.g.
> http://libvirt.org/formatdomain.html
> 
> . from this page it seems like using something like
>   <os>
>     <type arch='i686'>exe</type>
>     <init>/sbin/init</init>
>   </os>
> in the guest XML config should do the trick
Yes, setting arch='i686' should force the container to have a 32-bit
personality on x64_64. Likewise for ppc vs ppc64, s390 vs s390x, etc

Regards,
Daniel
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