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

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Date:	Fri, 04 May 2007 11:55:15 -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:
>> 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.
>>     
>
> That wasn't paravirt detection that was do I have permissions to load
> the gdt.
>   

Well, a paravirtualized ring0 kernel may still have special constraints
on how the gdt can be set up (page-aligned, read-only, etc).

> Basically we have two choices.   Either unconditionally demand
> that %cs %ds %es and %ss are loaded, and so we remove all descriptor
> loading.
>   

Yep.  At least if the boot-version is new enough to boot this way.  The
old native-boot path can reload as much as it likes.

>> 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();
>> 	}
>>     
>
> I think it is much better to test the boot_params.platform field where
> we care.  If the platform is a native x86 subarch we don't need
> a magic startup function.  If the platform is Xen or lguest
> we can at best copy our boot parameters and jump to their custom
> startup routines.
>   

Why not just treat them all in the same way?  Especially if we start
sweeping other non-virtual architectures like voyager/visws/etc into the
same mechanism.

My idea was that "goto native_boot" would jump to code which assumes
it's running on real hardware, where there's no problem reloading
gdt/segment registers, etc.

    J
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