From c74adfecd0b21ea84f7e98629549f7489c78edc9 Mon Sep 17 00:00:00 2001
From: Samuel Thibault <samuel.thibault@ens-lyon.org>
Date: Sat, 24 Mar 2012 03:01:01 +0100
Subject: Relocate kernel at bootup

Grub is not able to map us at high addresses. We can however make it load us at
low addresses (through the linker script mapping), then use segmentation to move
ourselves to high addresses, and switch to C which uses high addresses
(through _START definition).

* i386/ldscript: Force mapping kernel at 0x100000, regardless of the value
of _START.
* i386/i386at/boothdr.S (boot_entry): Set up initial segments to project the
bootstrap linear space to high addresses.
---
 i386/i386at/boothdr.S | 37 +++++++++++++++++++++++++++++++++++++
 i386/ldscript         |  1 +
 2 files changed, 38 insertions(+)

diff --git a/i386/i386at/boothdr.S b/i386/i386at/boothdr.S
index 45cd599..567851e 100644
--- a/i386/i386at/boothdr.S
+++ b/i386/i386at/boothdr.S
@@ -39,6 +39,19 @@ boot_hdr:
 #endif /* __ELF__ */
 
 boot_entry:
+	/* use segmentation to offset ourself. */
+	lgdt	boot_gdt_descr - KERNELBASE
+	ljmp	$8,$0f
+0:
+	movw	$0,%ax
+	movw	%ax,%ds
+	movw	%ax,%es
+	movw	%ax,%fs
+	movw	%ax,%gs
+	movw	$16,%ax
+	movw	%ax,%ds
+	movw	%ax,%es
+	movw	%ax,%ss
 
 	/* Switch to our own interrupt stack.  */
 	movl	$_intstack+INTSTACK_SIZE,%esp
@@ -80,3 +93,27 @@ iplt_done:
 
 	.comm	_intstack,INTSTACK_SIZE
 
+.align 16
+	.word 0
+boot_gdt_descr:
+	.word 3*8+7
+	.long boot_gdt - KERNELBASE
+.align 16
+boot_gdt:
+	/* 0 */
+	.quad 0
+	/* boot CS = 8 */
+	.word 0xffff
+	.word (-KERNELBASE) & 0xffff
+	.byte ((-KERNELBASE) >> 16) & 0xff
+	.byte 0x9a
+	.byte 0xcf
+	.byte ((-KERNELBASE) >> 24) & 0xff
+	/* boot DS = 8 */
+	.word 0xffff
+	.word (-KERNELBASE) & 0xffff
+	.byte ((-KERNELBASE) >> 16) & 0xff
+	.byte 0x92
+	.byte 0xcf
+	.byte ((-KERNELBASE) >> 24) & 0xff
+
diff --git a/i386/ldscript b/i386/ldscript
index bbf5698..a63ff77 100644
--- a/i386/ldscript
+++ b/i386/ldscript
@@ -13,6 +13,7 @@ SECTIONS
    */
   . = _START;
   .text           :
+  AT (0x00100000)
   {
     *(.text.start)
     *(.text .stub .text.* .gnu.linkonce.t.*)
-- 
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