linux-kernel - Re: [Xen-devel] Re: [GIT PULL] Xen APIC hooks (with io_apic

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Date:	Thu, 28 May 2009 02:13:50 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
Cc:	Avi Kivity <avi@...hat.com>,
	George Dunlap <George.Dunlap@...citrix.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Keir Fraser <keir.fraser@...citrix.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [Xen-devel] Re: [GIT PULL] Xen APIC hooks (with io_apic_ops)


* Dan Magenheimer <dan.magenheimer@...cle.com> wrote:

> > The Linux scheduler already supports multiple scheduling 
> > classes.  If we find that none of them will fit our needs, we'll 
> > propose a new one.  When the need can be demonstrated to be 
> > real, and the implementation can be clean, Linux can usually be 
> > adapted.
> 
> But that's exactly George and Jeremy's point.  KVM will eventually 
> require changes that clutter Linux for purposes that are relevant 
> only to a hypervisor.

That's wrong. Any such scheduler classes would also help: control 
groups, containers, vserver, UML and who knows what other isolation 
project. Many of such mechanisms are already implemented as well.

I rarely see any KVM-only feature in generic kernel code, and that's 
good.

Xen changes - especially dom0 - are overwhelmingly not about 
improving Linux, but about having some special hook and extra 
treatment in random places - and that's really bad.

I also find it pretty telling that you cut out the most important 
point of Avi's reply:

> > I think the Xen design has merit if it can truly make dom0 a 
> > guest -- that is, if it can survive dom0 failure.  Until then, 
> > you're just taking a large interdependent codebase and splitting 
> > it at some random point, but you don't get any stability or 
> > security in return.

that crucial question really has to be answered honestly and 
upfront.

	Ingo
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