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

To: Alexander Graf <agraf@xxxxxxx>
Subject: [Xen-devel] Re: [Qemu-devel] [PATCH 00/10] xen: pv domain support.
From: Avi Kivity <avi@xxxxxxxxxx>
Date: Sun, 05 Apr 2009 14:37:49 +0300
Cc: qemu-devel <qemu-devel@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, Gerd Hoffmann <kraxel@xxxxxxxxxx>, Laurent Vivier <Laurent@xxxxxxxxx>
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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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