[libvirt] libvirt arch detection on x86_64 host

[Daniel P. Berrange]

On Tue, Apr 14, 2009 at 09:23:35PM -0400, Gerry Reno wrote:
> Gerrit Slomma wrote:
> >Gerry Reno schrieb:
> >>
> >>I have a 64-bit host that is running a 32-bit OS (Fedora 10).
> >>(...)
> >># virsh nodeinfo
> >>CPU model:           i686
> >>(...)
> >>Shouldn't the host arch have been detected and identified as x86_64?

The host arch reflects the arch of the "hypervisor". In Xen where you
have a separate Hypervisor and Dom0 OS, you can in theory use a 64-bit
hypervisor and 32-bit Dom0 and still run 64-bit guests. In KVM, the
host OS *is* the hypervisor, so you have to run a 64-bit host OS if
you want 64-bit guests.

....unles you want to run really slow QEMU 64-bit emulation, but I'm
pretty sure you don't want todo that :-)

> >
> >As stated by you in the beginning: You are running a 32-bit OS.
> >You can't have more bit than the OS is providing you.
> >/usr/bin/qemu-system-x86_64 alwas shows x86_64, even on my Core Duo.
>
> Is it allowed to run x86_64 guest OS on x86_64 physical machine with x86 
> (32-bit) host OS?

Technically it is not the host OS arch that matters, but rather the 
hypervisor arch. Since KVM is built-in to the Linux host OS, you need
a 64-bit host OS. Really you don't want to rnu 32-bit host OS under
any circumstances. 32-bit has horrible memory limitations, horrible
register limitations and should be left to die an ungraceful death.
32-bit guests are fine if you so wish, but just say no to 32-bit hosts,
particularly since you have 64-bit CPUs

Daniel
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