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Re: [Xen-devel] HYBRID: PV in HVM container

To: Keir Fraser <keir@xxxxxxx>
Subject: Re: [Xen-devel] HYBRID: PV in HVM container
From: Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>
Date: Tue, 28 Jun 2011 09:49:27 +0100
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On Tue, 2011-06-28 at 09:35 +0100, Keir Fraser wrote:
> On 28/06/2011 09:30, "Ian Campbell" <Ian.Campbell@xxxxxxxxxx> wrote:
> > On Tue, 2011-06-28 at 08:46 +0100, Keir Fraser wrote:
> >> On 28/06/2011 02:51, "Mukesh Rathor" <mukesh.rathor@xxxxxxxxxx> wrote:
> >> 
> >> Well, maybe. But we now have HVM guests, PV guests, and PV-HVM guests. I'm
> >> not sure that adding explicitly HVM-PV guests as well isn't just a bloody
> >> mess.
> > 
> > Ideally this container could be used to accelerate existing 64 bit
> > guests (e.g. older distros running classic-Xen) unmodified (or at least
> > only with latent bugs fixed) too.
> There was a question mark over whether unmodified PV guests would tolerate
> running in ring 0, rather than entirely in ring 3. I believe we're confident
> it should work, and thus supporting classic-Xen guests should certainly be
> the aim.

A guest which does XENFEAT_supervisor_mode_kernel (and perhaps one or
two other XENFEATs) should work, but that was the primary source of the
latent bugs I was thinking of...

In particular the pvops kernel probably doesn't do all the right things
for XENFEAT_supervisor_mode_kernel, since it has never been run that
way, but it also doesn't advertise it via XEN_ELFNOTE_FEATURES so we can
at least detect when it is safe to enable the container from the builder

> > Getting something working with a modified guest seems like a useful
> > first step (to get to a working baseline) but I'm not sure it should be
> > the end goal.
> I certainly don't think we should commit such a thing without careful
> thought.



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