Linux has C macros and code to load the GTD after switching to Protected Mode. Using these greatly simplifies the assembler code
Signed-off-by: Graeme Russ graeme.russ@gmail.com --- arch/i386/cpu/cpu.c | 57 +++++++++++++++++++++++++++++ arch/i386/cpu/start.S | 96 ------------------------------------------------- 2 files changed, 57 insertions(+), 96 deletions(-)
diff --git a/arch/i386/cpu/cpu.c b/arch/i386/cpu/cpu.c index bd6aced..ae40384 100644 --- a/arch/i386/cpu/cpu.c +++ b/arch/i386/cpu/cpu.c @@ -37,6 +37,61 @@ #include <command.h> #include <asm/interrupt.h>
+/* Constructor for a conventional segment GDT (or LDT) entry */ +/* This is a macro so it can be used in initializers */ +#define GDT_ENTRY(flags, base, limit) \ + ((((base) & 0xff000000ULL) << (56-24)) | \ + (((flags) & 0x0000f0ffULL) << 40) | \ + (((limit) & 0x000f0000ULL) << (48-16)) | \ + (((base) & 0x00ffffffULL) << 16) | \ + (((limit) & 0x0000ffffULL))) + +/* Simple and small GDT entries for booting only */ + +#define GDT_ENTRY_32BIT_CS 2 +#define GDT_ENTRY_32BIT_DS (GDT_ENTRY_32BIT_CS + 1) +#define GDT_ENTRY_16BIT_CS (GDT_ENTRY_32BIT_DS + 1) +#define GDT_ENTRY_16BIT_DS (GDT_ENTRY_16BIT_CS + 1) + +/* + * Set up the GDT + */ + +struct gdt_ptr { + u16 len; + u32 ptr; +} __attribute__((packed)); + +static void reload_gdt(void) +{ + /* There are machines which are known to not boot with the GDT + being 8-byte unaligned. Intel recommends 16 byte alignment. */ + static const u64 boot_gdt[] __attribute__((aligned(16))) = { + /* CS: code, read/execute, 4 GB, base 0 */ + [GDT_ENTRY_32BIT_CS] = GDT_ENTRY(0xc09b, 0, 0xfffff), + /* DS: data, read/write, 4 GB, base 0 */ + [GDT_ENTRY_32BIT_DS] = GDT_ENTRY(0xc093, 0, 0xfffff), + /* 16-bit CS: code, read/execute, 64 kB, base 0 */ + [GDT_ENTRY_16BIT_CS] = GDT_ENTRY(0x109b, 0, 0x0ffff), + /* 16-bit DS: data, read/write, 64 kB, base 0 */ + [GDT_ENTRY_16BIT_DS] = GDT_ENTRY(0x1093, 0, 0x0ffff), + }; + static struct gdt_ptr gdt; + + gdt.len = sizeof(boot_gdt)-1; + gdt.ptr = (u32)&boot_gdt; + + asm volatile("lgdtl %0\n" \ + "movl $((2+1)*8), %%ecx\n" \ + "movl %%ecx, %%ds\n" \ + "movl %%ecx, %%es\n" \ + "movl %%ecx, %%fs\n" \ + "movl %%ecx, %%gs\n" \ + "movl %%ecx, %%ss" \ + : : "m" (gdt) : "ecx"); +} + + int cpu_init_f(void) { /* initialize FPU, reset EM, set MP and NE */ @@ -51,6 +106,8 @@ int cpu_init_f(void)
int cpu_init_r(void) { + reload_gdt(); + /* Initialize core interrupt and exception functionality of CPU */ cpu_init_interrupts (); return 0; diff --git a/arch/i386/cpu/start.S b/arch/i386/cpu/start.S index 7def8de..3cea04b 100644 --- a/arch/i386/cpu/start.S +++ b/arch/i386/cpu/start.S @@ -100,53 +100,6 @@ mem_init_ret: jmp get_mem_size get_mem_size_ret:
- /* - * We are now in 'Flat Protected Mode' and we know how much memory - * the board has. The (temporary) Global Descriptor Table is not - * in a 'Safe' place (it is either in Flash which can be erased or - * reprogrammed or in a fail-safe boot-strap image which could be - * over-written). - * - * Move the final gdt to a safe place (top of RAM) and load it. - * This is not a trivial excercise - the lgdt instruction does not - * have a register operand (memory only) and we may well be - * running from Flash, so self modifying code will not work here. - * To overcome this, we copy a stub into upper memory along with - * the GDT. - */ - - /* Reduce upper memory limit by (Stub + GDT Pointer + GDT) */ - subl $(end_gdt_setup - start_gdt_setup), %eax - - /* Copy the GDT and Stub */ - movl $start_gdt_setup, %esi - movl %eax, %edi - movl $(end_gdt_setup - start_gdt_setup), %ecx - shrl $2, %ecx - cld - rep movsl - - /* write the lgdt 'parameter' */ - subl $(jmp_instr - start_gdt_setup - 4), %ebp - addl %eax, %ebp - movl $(gdt_ptr - start_gdt_setup), %ebx - addl %eax, %ebx - movl %ebx, (%ebp) - - /* write the gdt address into the pointer */ - movl $(gdt_addr - start_gdt_setup), %ebp - addl %eax, %ebp - movl $(gdt - start_gdt_setup), %ebx - addl %eax, %ebx - movl %ebx, (%ebp) - - /* Save the return address */ - movl $load_gdt_ret, %ebp - - /* Load the new (safe) Global Descriptor Table */ - jmp *%eax - -load_gdt_ret: /* Check we have enough memory for stack */ movl $CONFIG_SYS_STACK_SIZE, %ecx cmpl %ecx, %eax @@ -221,52 +174,3 @@ die: hlt blank_idt_ptr: .word 0 /* limit */ .long 0 /* base */ - -.align 4 -start_gdt_setup: - lgdt gdt_ptr -jmp_instr: - jmp *%ebp - -.align 4 -gdt_ptr: - .word 0x30 /* limit (48 bytes = 6 GDT entries) */ -gdt_addr: - .long gdt /* base */ - - /* The GDT table ... - * - * Selector Type - * 0x00 NULL - * 0x08 Unused - * 0x10 32bit code - * 0x18 32bit data/stack - * 0x20 16bit code - * 0x28 16bit data/stack - */ - -.align 4 -gdt: - .word 0, 0, 0, 0 /* NULL */ - .word 0, 0, 0, 0 /* unused */ - - .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) */ - - .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) */ - - .word 0xFFFF /* 64kb */ - .word 0 /* base address = 0 */ - .word 0x9b00 /* data read/write */ - .word 0x0010 /* granularity = 1 (+5th nibble of limit) */ - - .word 0xFFFF /* 64kb */ - .word 0 /* base address = 0 */ - .word 0x9300 /* data read/write */ - .word 0x0010 /* granularity = 1 (+5th nibble of limit) */ -end_gdt_setup: