[PATCH] access gdtdesc on segment 0 unconditionally (Re: [PATCH] i386-qemu port)

[Robert Millan <rmh@aybabtu.com>]

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In this line of code in real_to_prot():

	DATA32  ADDR32  lgdt    %cs:gdtdesc

GAS generates an absolute address for `gdtdesc' (not relative to segment),
and so for the code to work %cs must be zero.  In current usage of
real_to_prot(), %cs is always zero because we jump to 0x0:0x82xx early on.

However, in other situations this is not possible.  On i386-qemu, before
moving to i386 mode the code we're running is in the 0xf0000-0x100000
range, which is inaccessible from segment 0.

This patch changes this to access gdtdesc from segment 0 unconditionally.

-- 
Robert Millan

  The DRM opt-in fallacy: "Your data belongs to us. We will decide when (and
  how) you may access your data; but nobody's threatening your freedom: we
  still allow you to remove your data and not access it at all."

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2009-06-22  Robert Millan  <rmh.grub@aybabtu.com>

	* kern/i386/pc/startup.S (real_to_prot): Access `gdtdesc' using
	segment 0x0 unconditionally, because the reference generated by
	GAS is an absolute address.

Index: kern/i386/pc/startup.S
===================================================================
--- kern/i386/pc/startup.S	(revision 2353)
+++ kern/i386/pc/startup.S	(working copy)
@@ -1,6 +1,6 @@
 /*
  *  GRUB  --  GRand Unified Bootloader
- *  Copyright (C) 1999,2000,2001,2002,2003,2005,2006,2007,2008 Free Software Foundation, Inc.
+ *  Copyright (C) 1999,2000,2001,2002,2003,2005,2006,2007,2008,2009 Free Software Foundation, Inc.
  *
  *  GRUB is free software: you can redistribute it and/or modify
  *  it under the terms of the GNU General Public License as published by
@@ -313,13 +313,9 @@
 	cli
 
 	/* load the GDT register */
-#ifdef APPLE_CC
-	mov %cs, %ax
-	mov %ax, %ds
-	DATA32	ADDR32	lgdt	gdtdesc
-#else
-	DATA32	ADDR32	lgdt	%cs:gdtdesc
-#endif
+	xorw	%ax, %ax
+	movw	%ax, %ds
+	DATA32	ADDR32	lgdt	%ds:gdtdesc
 
 	/* turn on protected mode */
 	movl	%cr0, %eax