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Re: [Xen-devel] Linux Xen Balloon Driver Improvement (Draft 2)

On Mon, 27 Oct 2014, Wei Liu wrote:
> On Mon, Oct 27, 2014 at 05:29:16PM +0000, David Vrabel wrote:
> > > I don't think there's terribly tight linkage between hypervisor side
> > > change and guest side change.
> > 
> > I don't see how you can think this unless you also have a design for the
> > hypervisor side.
> > 
> Because the basic requirement for this design is to not rely on
> hypervisor side feature, so that we can have it worked on older
> hypervisor as well. And by far the proposed design seems to stick to
> that principle well.

Wei, you do have a design for the hypervisor side changes: the design is
to make no hypervisor side changes. Maybe you should add:

## Hypervisor design for Linux Balloon Driver Compaction

No hypervisor changes required. The guest is going to make use of the
existing XENMEM_exchange interface. The Linux feature should work on any
hypervisor since Xen XXX.

> > I do not want a situation were effective and efficient host
> > defragmentation requires balloon driver changes to avoid a regression.

As you know there are non-Linux domU and dom0 out there that people use.
Older Linux and Xen versions mix and match quite well too. I think that
Linux side ballooning and Xen side defragmentation should be required to
be independent. Specifying that we don't need any Xen side changes to
implement Linux compaction is a good way to do that.

Also it is not common in the Linux community to request design
documents.  I don't think we should make them mandatory now. If we
really want to go down that path within the Xen community, we should
talk about it at the next Hackathon/Meeting and decide with the support
of the majority of the maintainers.

Personally, I am not in favor.

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