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

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Message-ID: <463B55F6.5030306@zytor.com>
Date:	Fri, 04 May 2007 08:49:10 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andi Kleen <ak@...e.de>, Chris Wright <chrisw@...s-sol.org>,
	Jeremy Fitzhardinge <jeremy@...p.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

Rusty Russell wrote:
> 
> It was a PITA because we can't just load $0x18 into %ss: it has to be
> $0x19.  I started doing it (read %cs, mask off bottom 2 bits, orl
> __BOOT_DS), but between that extra code and the extra code to then
> change segments back...  well, we have to skip the cli anyway so it was
> easy to skip them all.
> 
> I think Jeremy said Xen doesn't start on the boot descriptors either.  I
> have to wonder why we don't just run on the boot descriptors forever
> (#define __KERNEL_DS __BOOT_DS etc)?
> 

You can't do that once you define your own layout, or you introduce a
nasty intermodule dependency which shouldn't exist.

The rule should be:

- You MUST NOT reload segments before LGDT.
  (One can possibly get away with declaring that DS = CS+8 by
  protocol and load dynamic values, but that's not a good thing.)
- After LGDT, you MUST load your own segment values.

Anything else is just broken.

	-hpa
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