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

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Date:	Fri, 04 May 2007 10:25:30 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	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>,
	"H. Peter Anvin" <hpa@...or.com>,
	lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 3/3] boot bzImages under paravirt

Eric W. Biederman wrote:
> Hmm. If we made that:
> 	mov	%cs, %eax
> 	add	$0x10, %eax
> 	mov	%eax, %ds
>
> That is likely even backwards compatible.  If you don't mind having a fixed
> offset between the code and the data segments.  As I recall code and data
> are not interchangeable.
>   

Yes, that's just bogus thinko on my part.

> I'm trying to remember the reason for the reloads.
>
> As I recall loadlin intercepts code32_start from head.S and so it
> can do things just after we have switched to protected mode.  Because
> historically we didn't load the segments before this jump loadlin had
> to do it.  The code of loadlin appears to reload all of the segments
> just like head.S does and then not touch them.
>
> My two bootloaders that enter the kernel at the 32bit entry point already
> load the segments as well.
>
> Gujin looks like it loads just %es and %ds.
>   

We should be able to make do with that until we've got our own gdt.

> It is hard to tell with elilo what it sets up, it preserves the
> descriptors from EFI, but sets up a linux boot protocol gdt.
>   

? But the cached descriptors are still the EFI ones?

> Since setup.S finally does the right thing in loading segment
> registers.  It looks to me like we need to sit down and document
> the 32bit kernel interface, as it is today, and then extend
> things to just replicate %ds into the other segments, and kill
> any lss instructions.
>   

Yes.

> At the same time we are doing this it would be good to drop our
> boot protocol version into an ELF note so people booting vmlinux
> can discover when we have relaxed various restrictions and which
> fields in the real mode data we support.
>   

Do you mean the have the kernel expose its max supported version for
bootloaders to inspect?

    J
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