Re: [Qemu-devel] [PATCH 00/10] xen: pv domain support.

[Alexander Graf]

On 05.04.2009, at 10:48, Avi Kivity wrote:

> Alexander Graf wrote:
> > One idea I had for full virtualization in a Xen environment would  
> > be an PV vmenter/vmexit framework - either by implementing a  
> > completely new abstraction or simple traps for privileged  
> > operations like VMRUN.
> >
> > That way we could have a kvm that talks to xen for the VM,  
> > rendering kvm useful on Xen dom0s, giving people the best of both  
> > worlds.
> >
> > That was only one of the ideas that came up while talking to people  
> > why running kvm on xen isn't as easy as just recompiling :-). Would  
> > you think of such a thing as useful?
>
> Why would anyone want to do that?  If you've got Xen running, just  
> start up a Xen guest.

I'm not saying it's a great idea - that's why I didn't even consider  
to develop it yet :-).

Basically it would solve two problems:

1) Migration path. If you could already use KVM on a Xen host, you  
could have Xen PV guests and KVM guests in parallel, easing migration  
to KVM for customers.

2) Alternative to HVM. That's how this came up from Gerd's mail. We do  
have KVM support in upstream qemu, but we don't have Xen HVM support.  
That way you could use the same binary for all your needs. Admittedly,  
it might make more sense to just implement HVM support :-).

Again, I just like talking to others about random ideas I have and  
this was one. I don't think it's worth it - IMHO it'd be more useful  
to create an in-kernel xen-like module that exposes Xen PV  
functionality, so you get all the PV benefits without the performance  
hit from full virtualization and duplication of code.

Alex