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Re: [Xen-API] New API Document and C Bindings

To: Ewan Mellor <ewan@xxxxxxxxxxxxx>
Subject: Re: [Xen-API] New API Document and C Bindings
From: Jim Fehlig <jfehlig@xxxxxxxxxx>
Date: Thu, 14 Sep 2006 15:36:35 -0600
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Ewan Mellor wrote:
On Tue, Sep 12, 2006 at 07:16:40PM -0400, Stefan Berger wrote:

Also I have a question regarding domain-0. How will it be represented? Is
it a VM - the fact that 'guest' is written in the description of the VM
class makes me think that this might not be the case.

That's a very good question.  My instinct is to say that dom-0 shouldn't
be part of the list of domains, and that it should be considered part of
the infrastructure.  When we have driver domains, and HVM stub domains,
there will be many of these domains, representing different parts of the
infrastructure, and it seems to me that these are not the same as
"guests" or "VMs".  A VM can be rebooted, migrated (possibly), each time
keeping the same VM, but ending up with a different domain.  A VM is
ultimately the reason that users are running Xen, and the thing that
makes it useful.  For this reason, I don't think that domain 0 is a VM.

On the other hand, these things are still useful entities -- you might
want to monitor the CPU cost due to each of them, tweak their scheduling
parameters, and so on.  So perhaps they are close enough to being a VM
that we should put them in there, and cope with the slightly special
nature of them as best we can.

What do people think?

Even though dom0 is part of the infrastructure (or as Gareth pointed out akin to a HMC), it still needs to be managed and many of the management functions are no different from 'normal' guests. VCPUs can be hot-plugged and pinned to PCPUs, memory can be added / removed, etc. From the perspective of adding / removing resources, dom0 is no different than any other VM.

Driver and stub domains appear to be implementation details. A management app shouldn't have to know or care about them, at least not in the same regard as normal VMs. That said, I still see the need to be able to control the resources of such infrastructure domains - again such as restricting the PCPUs these domains can run on.

Guess I haven't said anything new - just added to the uncertainty of how we are going to handle these infrastructure domains that require a subset of the management functionality of their first-class brethren :-).


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