[TriLUG] Xen: Identifying Host from Guest

[Greg Cox]

On Thu, 10 May 2007, Robert Dale wrote:

> I think the problem here is that you're trying to break the separation
> of concerns.  A guest really shouldn't care or know who is hosting it.

I disagree, and I think that's a short-sighted approach.  I think it's
quite valid for a guest to be able to ask both "Am I a domU, a domO, or
plain vanilla?" and, as an extension, "Oh, I'm a domU?  Who's my host?"

Let's say I have a domU that, through the magic of the domU's filesystem
being on a snapmirrored filer volume and good networking convergence,
can end up being cold-failed off to a DR site.  Some backup software,
which magically survives this meteor strike, calls up the agent on the
relocated domU and says, "Time for your backup!"  I would love to have
the agent do some noodling on its own and say, "Aw, mom!  I'm over at
Timmy's, and it's no longer a bored GbE utility network between us,
it's single 100Mb MetroE, and I bet production ops are more important
than weekly fulls, so, I want to say No to everything but Oracle
archivelogs."

> The host already knows what guests it has, so query that.  You could
> build or use a management app to control all this.  Then if your guest
> really needs to know where it is, it would then query the manager.

Let's say I have a domU that wants to run a hog process, as long as it
won't affect other domU's on the dom0.  Having it figure out "I'm on
node3, let me poll node3 for how my same-box-brethren are running"
shouldn't be so hard.  Some sort of readonly(-if-dom0-allows) out-of-band
access to the hypervisor.  I don't even necessarily want this available
by default, but not having access to, say, uname of the host (even if
that's not guaranteed unique), is potentially limiting.

Both of these scenarios could be worked out through other means, I fully
admit, so, nobody really needs to come up with "well you could do it like
THIS."  It just seems like there are cases where knowing something about
your dom0 could make admin life easier.