Date:	Wed, 03 Jun 2009 18:37:47 -0400
From:	Bill Davidsen <davidsen@....com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	George Dunlap <george.dunlap@...citrix.com>,
	David Miller <davem@...emloft.net>,
	"jeremy@...p.org" <jeremy@...p.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	"avi@...hat.com" <avi@...hat.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>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	"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
Subject: Re: Xen is a feature

Thomas Gleixner wrote:
> On Wed, 3 Jun 2009, Bill Davidsen wrote:
>   
>> Thomas Gleixner wrote:
>>     
>>> Aside of the paravirt, which seems to expand through arch/x86 like a
>>> hydra, the new patches sprinkle "if (xen_...)" all over the
>>> place. These extra xen dependencies are no improvement, they are a
>>> royal pain in the ... They are sticky once they got merged simply
>>> because the hypervisor relies on them and we need to provide
>>> compatibility for a long time.
>>>
>>>       
>> Wait, let's not classify something as "no improvement" when you mean "I don't
>> need it."
>>     
>
> It's not about "I don't need it.". It's about having Xen dependencies
> in the code all over the place which make mainatainence harder. I have
> to balance the users benefit (xen dom0 support) vs. the impact on
> maintainability and the restrictions which are going to be set almost
> in stone by merging it.
>
>   
>> Let's stick to technical issues, and not deny that there are a number of users
>> who really will have expanded capability. The technical points are valid, but
>> as a former and probable future xen (CentOS) user, so are the benefits.
>>     
>
> Refusing random "if (xen...)" dependencies is a purely technical
> decision. I have said more than once that I'm not against merging dom0
> in general, I'm just frightened by the technical impact of a defacto
> ABI which we swallow with it.
>
>   
I was referring to your "no benefit" comment, I don't dispute the 
technical issues. I think the idea of moving the hypervisor into the 
kernel and letting xen folks do the external parts as they please.

> We have enough problems with real silicon and BIOS/ACPI already, why
> should we add artifical and _avoidable_ virtual silicon horror ?
>   

I guess my point wasn't clear, sorry, it's just that I felt as though 
the features lacking KVM (old/small/BIOS-limited CPUs) might be hidden 
in the smoke due to the technical issues.

-- 
Bill Davidsen <davidsen@....com>
  Even purely technical things can appear to be magic, if the documentation is
obscure enough. For example, PulseAudio is configured by dancing naked around a
fire at midnight, shaking a rattle with one hand and a LISP manual with the
other, while reciting the GNU manifesto in hexadecimal. The documentation fails
to note that you must circle the fire counter-clockwise in the southern
hemisphere.


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