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Date: Fri, 13 Aug 2010 14:10:48 +0100
From: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Subject: Re: [Qemu-devel] [PATCH 03/15] xen: Add a new target to qemu:
	target-xen
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To: Blue Swirl <blauwirbel@gmail.com>
Cc: Anthony Perard <anthony.perard@citrix.com>, "xen-devel@lists.xensource.com" <xen-devel@lists.xensource.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Stefano Stabellini <Stefano.Stabellini@eu.citrix.com>

On Thu, 12 Aug 2010, Blue Swirl wrote:
> On Thu, Aug 12, 2010 at 2:09 PM,  <stefano.stabellini@eu.citrix.com> wrote:
> > From: Anthony PERARD <anthony.perard@citrix.com>
> >
> > This patch adds a new Xen device model target to Qemu, called
> > target-xen.
> 
> I don't understand why it would be a target, QEMU calls CPU
> architectures targets. Isn't it possible to have Xen for Sparc, PPC or
> ARM? It should really be just a machine, not copy&paste from x86
> target.
> 

It is not possible to have Xen device models for Sparc, PPC or ARM: the
only architecture that supports Xen HVM guests is x86 and x86_64.
Also most of the x86 copy and paste are definitions or stubs that
haven't really changed in a very long time.


> > +/*
> > + * This next section was clone-and-hacked from the version in exec.c
> > + * :-(.  But the exec.c version is full of tcg-specific stuff and
> > + * assumptions about phys_ram_base.
> 
> Then fix those assumptions and introduce xen specific hooks, like KVM.
> 

That comment goes back to the very first qemu-xen integration, it is not
so relevant anymore.
I don't know kvm hooks well enough to be sure that something similar
could work well for Xen too and honestly I don't see many benefits in
doing so.
In particular I am afraid that taking that route might cause a much
heavier impact on common code and APIs and ultimately could cause more
problems than it solves. As you can see the current approach has the
benefit of being self-contained and considering that the xen device
model use case works only on x86, doesn't do any cpu emulation and needs
a specific set of emulated hardware, I think it makes sense to have its
own target.