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Date: Fri, 04 May 2007 08:49:10 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Rusty Russell <rusty@rustcorp.com.au>
CC: "Eric W. Biederman" <ebiederm@xmission.com>, Andi Kleen <ak@suse.de>,
       Chris Wright <chrisw@sous-sol.org>,
       Jeremy Fitzhardinge <jeremy@goop.org>, Zachary Amsden <zach@vmware.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       lkml - Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH 3/3] boot bzImages under paravirt
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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