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 (file)
--- a/arch/x86/cpu/interrupts.c
+++ b/arch/x86/cpu/interrupts.c
@@ -174,7 +174,7 @@ struct desc_ptr {
        unsigned short segment;
 } __packed;
 
        unsigned short segment;
 } __packed;
 

-struct idt_entry idt[256] __attribute__((aligned(16)));
+struct idt_entry idt[256] __aligned(16);

 
 struct desc_ptr idt_ptr;
 
 
 struct desc_ptr idt_ptr;
 

diff --git a/arch/x86/cpu/start16.S b/arch/x86/cpu/start16.S
index 33e53cd..cc393ff 100644 (file)
--- 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 */
 
        .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
         *
         *       Selector       Type
         *       0x00           NULL


@@ -94,17 +98,46 @@ gdt_ptr:
         *       0x10           32bit code
         *       0x18           32bit data/stack
         */
         *       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 */