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

[Avi Kivity <avi@xxxxxxxxxx>]

Alexander Graf wrote:
> 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.
I like this, of course, but we have a path through Xenner.  Maybe this 
(kvm-on-xen) path will be easier to take.
> 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 :-).
I was under the impression that this is underway.

> 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.
With npt/ept pv performance might be higher running under kvm+xenner 
than with software-only Xen by letting the guest kernel access 
pagetables directly.  Though Gerd had some issues with 64-bit guests 
IIRC, which is a pity since it's there that the pv performance hit is 
greatest.

--
error compiling committee.c: too many arguments to function


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