linux-kernel - Re: [RFC PATCH 3/3] boot bzImages under paravirt

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Date:	Fri, 04 May 2007 11:22:30 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Rusty Russell <rusty@...tcorp.com.au>, Andi Kleen <ak@...e.de>,
	Chris Wright <chrisw@...s-sol.org>,
	Zachary Amsden <zach@...are.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 3/3] boot bzImages under paravirt

Eric W. Biederman wrote:
>> Gujin seems to have a near-zero user community, so if they have to rev
>> their code it wouldn't be a big deal (the author keeps trying to push
>> some crack-smoking "Gujin native" patches into the kernel, too), 
>> breaking ELILO would hurt anyone using Intel Macs.
>>     
>
> I'm thinking we just make the code start.
> startup_32:
> 	movl	%cs, %eax
>         testl   $3, %eax
>         jnz     1f
>   

I'm not really happy about using this as a way to distinguish paravirt
from non-paravirt in general.  At some point we're going to be running
paravirt kernels in ring0 within a VT/SVM container - but they'll still
be completely paravirtualized kernels.

I think a better approach is to just do it purely based on the boot
params platform field.  Ie, something along the lines of:

	if (boot_params.version < new_enough)
		goto native_boot;
	else {
		for (int i = 0; i < nplatforms; i++)
			if (boot_params.platform == platforms[i].id)
				goto *platforms[i].startup
		panic();
	}


> But that won't work if we want to support relocatability.
> Because we can't load a gdt if we don't know where we are.
>
> To find out where we are we need %ss and %ds, at which point
> we might as well assume we have %es to.
>   

Yes, we won't make it far without ss and ds, and while we could avoid
string instructions, you'd have to be a pretty short-sighted bootloader
author to set ss and ds without also setting es.

> So be it then.  The next rev of the boot protocol gets to be partially
> incompatible, and we just assume that %cs, %ds, %es, %ss meet our
> basic requirements.  I'm pretty certain from what I saw only Gujin
> is going to suffer  :(
>   

I missed what Gujin does wrong here?


    J
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