linux-kernel - Re: Xen is a feature

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Date:	Tue, 02 Jun 2009 21:59:19 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	George Dunlap <george.dunlap@...citrix.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Miller <davem@...emloft.net>,
	"jeremy@...p.org" <jeremy@...p.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	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

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.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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