From 58c7a6751dd0978172741503b4dc0c6b23a60b33 Mon Sep 17 00:00:00 2001
From: Graeme Russ <graeme.russ@gmail.com>
Date: Mon, 19 Dec 2011 14:26:18 +1100
Subject: x86: Tweak IDT and GDT for alignment and readability

--
Changes for v2:
 - Renamed to better reflect nature of changes

diff --git a/arch/x86/cpu/interrupts.c b/arch/x86/cpu/interrupts.c
index e0958eb..43ec3f8 100644
--- a/arch/x86/cpu/interrupts.c
+++ b/arch/x86/cpu/interrupts.c
@@ -174,7 +174,7 @@ struct desc_ptr {
 	unsigned short segment;
 } __packed;
 
-struct idt_entry idt[256] __attribute__((aligned(16)));
+struct idt_entry idt[256] __aligned(16);
 
 struct desc_ptr idt_ptr;
 
diff --git a/arch/x86/cpu/start16.S b/arch/x86/cpu/start16.S
index 33e53cd..cc393ff 100644
--- a/arch/x86/cpu/start16.S
+++ b/arch/x86/cpu/start16.S
@@ -86,7 +86,11 @@ gdt_ptr:
 	.word	0x20		/* limit (32 bytes = 4 GDT entries) */
 	.long	BOOT_SEG + gdt	/* base */
 
-	/* The GDT table ...
+/* Some CPUs are picky about GDT alignment... */
+.align 16
+gdt:
+	/*
+	 * The GDT table ...
 	 *
 	 *	 Selector	Type
 	 *	 0x00		NULL
@@ -94,17 +98,46 @@ gdt_ptr:
 	 *	 0x10		32bit code
 	 *	 0x18		32bit data/stack
 	 */
+	/* The NULL Desciptor - Mandatory */
+	.word	0x0000		/* limit_low */
+	.word	0x0000		/* base_low */
+	.byte	0x00		/* base_middle */
+	.byte	0x00		/* access */
+	.byte	0x00		/* flags + limit_high */
+	.byte	0x00		/* base_high */
 
-gdt:
-	.word	0, 0, 0, 0	/* NULL  */
-	.word	0, 0, 0, 0	/* unused */
+	/* Unused Desciptor - (matches Linux) */
+	.word	0x0000		/* limit_low */
+	.word	0x0000		/* base_low */
+	.byte	0x00		/* base_middle */
+	.byte	0x00		/* access */
+	.byte	0x00		/* flags + limit_high */
+	.byte	0x00		/* base_high */
 
-	.word	0xFFFF		/* 4Gb - (0x100000*0x1000 = 4Gb) */
-	.word	0		/* base address = 0 */
-	.word	0x9B00		/* code read/exec */
-	.word	0x00CF		/* granularity = 4096, 386 (+5th nibble of limit) */
+	/*
+	 * The Code Segment Descriptor:
+	 * - Base   = 0x00000000
+	 * - Size   = 4GB
+	 * - Access = Present, Ring 0, Exec (Code), Readable
+	 * - Flags  = 4kB Granularity, 32-bit
+	 */
+	.word	0xffff		/* limit_low */
+	.word	0x0000		/* base_low */
+	.byte	0x00		/* base_middle */
+	.byte	0x9b		/* access */
+	.byte	0xcf		/* flags + limit_high */
+	.byte	0x00		/* base_high */
 
-	.word	0xFFFF		/* 4Gb - (0x100000*0x1000 = 4Gb) */
-	.word	0x0		/* base address = 0 */
-	.word	0x9300		/* data read/write */
-	.word	0x00CF		/* granularity = 4096, 386 (+5th nibble of limit) */
+	/*
+	 * The Data Segment Descriptor:
+	 * - Base   = 0x00000000
+	 * - Size   = 4GB
+	 * - Access = Present, Ring 0, Non-Exec (Data), Writable
+	 * - Flags  = 4kB Granularity, 32-bit
+	 */
+	.word	0xffff		/* limit_low */
+	.word	0x0000		/* base_low */
+	.byte	0x00		/* base_middle */
+	.byte	0x93		/* access */
+	.byte	0xcf		/* flags + limit_high */
+	.byte	0x00		/* base_high */
-- 
cgit v0.10.2