[Gllug] RHEL Virtualisation Puzzle

[Daniel P. Berrange]

On Tue, Jul 29, 2008 at 10:44:20AM +0100, Stephen Nelson-Smith wrote:
> Hi,
> 
> I have an x86_64 RHEL 5 update 2 machine, subscribed to the
> virtualisation channel.  I installed a 32 bit Centos 5 guest over
> http, using virt-install.
> 
> I'm slightly confused by conflicting information.  The Redhat
> documentation (Virtualisation Guide 5.2) indicates quite clearly that
> I cannot run a paravirtualised i686 guest on an x86_64 platform.  It
> also indicates, and I have read elsewhere, that to install a fully
> virtualised system I need to boot from installation media.  I
> categorically didn't boot off a CD/DVS or an ISO on the filesystem.  I
> just used virt-install, and pointed it at my rpm repository.  I don't
> recall whether I specified the -p flag, but I think the default is
> paravirtualisation, isn't it?

Yes, paravirtualization is the  default. So if you didn't explicitly
ask for HVM, then it would have done a paravirt install.

> I don't have access to the machine for a few days but my mind is
> puzzled.  Is it likely that my 32 bit domain is paravirtualised?  And
> if so, is the Redhat documentation out of date?  Or is it HVM?  In
> which case, is the Redhat documentation incorrect?

In RHEL5, update 1  attempting to run 32-bit paravirt guests  on a 64-bit
host will result in random/unpredictable crashes of the guest. It may
work fine for weeks, and then crash & burn horribly for weeks. This is
why we say it is unsupported in RHEL-5.1 - if its working for you it is
just pure luck and you will run out of luck eventually.

In RHEL5, update 2  the basic functionality of running 32-bit paravirt
guests on a 64-bit host will work, and run reliably. We still indicate
that it is considered tech-preview though, because save/restore/migrate
does not work in a 32-on-64 environment.

NB, 32-bit *fully* virtualized guests will always work on a 64-bit host
since RHEL-5, GA. The limitations only affect paravirt guests, or fullvirt
guests using the paravirt add-on drivers.

Daniel
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