Message-ID: <20090607091349.GA26897@elte.hu>
Date:	Sun, 7 Jun 2009 11:13:49 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Avi Kivity <avi@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	George Dunlap <george.dunlap@...citrix.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Miller <davem@...emloft.net>,
	"jeremy@...p.org" <jeremy@...p.org>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Keir Fraser <Keir.Fraser@...citrix.com>,
	"gregkh@...e.de" <gregkh@...e.de>,
	"kurt.hackel@...cle.com" <kurt.hackel@...cle.com>,
	Ian Pratt <Ian.Pratt@...citrix.com>,
	"xen-users@...ts.xensource.com" <xen-users@...ts.xensource.com>,
	ksrinivasan <ksrinivasan@...ell.com>,
	"EAnderson@...ell.com" <EAnderson@...ell.com>,
	"wimcoekaerts@...mekes.net" <wimcoekaerts@...mekes.net>,
	Stephen Spector <stephen.spector@...rix.com>,
	"jens.axboe@...cle.com" <jens.axboe@...cle.com>,
	"npiggin@...e.de" <npiggin@...e.de>
Subject: Re: Xen is a feature


* Avi Kivity <avi@...hat.com> wrote:

> Linus Torvalds wrote:
>> The point? Xen really is horribly badly separated out. It gets way more 
>> incestuous with other systems than it should. It's entirely possible 
>> that this is very fundamental to both paravirtualization and to 
>> hypervisor behavior, but it doesn't matter - it just measn that I can 
>> well see that Xen is a f*cking pain to merge.
>>
>> So please, Xen people, look at your track record, and look at the 
>> issues from the standpoint of somebody merging your code, rather 
>> than just from the standpoint of somebody who whines "I want my 
>> code to be merged".
>>
>> IOW, if you have trouble getting your code merged, ask yourself 
>> what _you_ are doing wrong.
>
> There is in fact a way to get dom0 support with nearly no changes 
> to Linux, but it involves massive changes to Xen itself and 
> requires hardware support: run dom0 as a fully virtualized guest, 
> and assign it all the resources dom0 can access.  It's probably a 
> massive effort though.
>
> I've considered it for kvm when faced with the "I want a thin 
> hypervisor" question: compile the hypervisor kernel with PCI 
> support but nothing else (no CONFIG_BLOCK or CONFIG_NET, no device 
> drivers), load userspace from initramfs, and assign host devices 
> to one or more privileged guests.  You could probably run the host 
> with a heavily stripped configuration, and enjoy the slimness 
> while every interrupt invokes the scheduler, a context switch, and 
> maybe an IPI for good measure.

This would be an acceptable model i suspect, if someone wants a 
'slim hypervisor'.

We can context switch way faster than we handle IRQs. Plus in a 
slimmed-down config we could intentionally slim down aspects of the 
scheduler as well, if it ever became a measurable performance issue. 
The hypervisor would run a minimal user-space and most of the 
context-switching overhead relates to having a full-fledged 
user-space with rich requirements. So there's no real conceptual 
friction between a 'lean and mean' hypervisor and a full-featured 
native kernel.

This would certainly be an utterly clean design, and it would be 
interesting to see a Linux/Xen + Linux/Dom0 combo engineered in such 
a way - if people really find this layered kernel approach 
interesting. So the door is not closed to dom0 at all - but it has 
to be designed cleanly without messing up the native kernel.

	Ingo
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