Currently, the GDT is either located in FLASH or in the non-relocated U-Boot image in RAM. Both of these locations are unsafe as those locations can be erased during a U-Boot update. Move the GDT into the highest available memory location and relocate U-Boot to just below it --- cpu/i386/start.S | 110 +++++++++++++++++++++++++++++++++++++++++++++++++--- cpu/i386/start16.S | 28 +++---------- 2 files changed, 111 insertions(+), 27 deletions(-)
diff --git a/cpu/i386/start.S b/cpu/i386/start.S index e1d4492..8874470 100644 --- a/cpu/i386/start.S +++ b/cpu/i386/start.S @@ -49,13 +49,8 @@ _i386boot_start: movl %eax, %cr0 wbinvd
- lidt blank_idt_ptr - - /* Tell 32-bit code it is being entered from */ + /* Tell 32-bit code it is being entered from an in-RAM copy */ movw $0x0000, %bx - - /* Flush the prefetch queue */ - jmp _start _start: /* This is the 32-bit cold-reset entry point */
@@ -67,6 +62,9 @@ _start: movw %ax,%es movw %ax,%ss
+ /* Clear the interupt vectors */ + lidt blank_idt_ptr + /* * Skip low-level board and memory initialization if not starting * from cold-reset. This allows us to do a fail safe boot-strap @@ -102,6 +100,53 @@ 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-ridden). + * + * 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 @@ -172,3 +217,56 @@ stack_ok: die: hlt jmp 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: diff --git a/cpu/i386/start16.S b/cpu/i386/start16.S index 1caa686..3e8b2cc 100644 --- a/cpu/i386/start16.S +++ b/cpu/i386/start16.S @@ -44,11 +44,9 @@ board_init16_ret: movl %eax, %cr0 wbinvd
- /* load the descriptor tables */ -o32 cs lidt blank_idt_ptr + /* load the temporary Global Descriptor Table */ o32 cs lgdt gdt_ptr
- /* Now, we enter protected mode */ movl %cr0, %eax orl $1,%eax @@ -70,13 +68,13 @@ code32start: .long _start /* offset */ .word 0x10 /* segment */
-.global blank_idt_ptr -blank_idt_ptr: - .word 0 /* limit */ - .long 0 /* base */ - +/* + * The following Global Descriptor Table is just enough to get us into + * 'Flat Protected Mode' - It will be discarded as soon as the final + * GDT is setup in a safe location in RAM + */ gdt_ptr: - .word 0x30 /* limit (48 bytes = 6 GDT entries) */ + .word 0x20 /* limit (32 bytes = 4 GDT entries) */ .long BOOT_SEG + gdt /* base */
/* The GDT table ... @@ -86,8 +84,6 @@ gdt_ptr: * 0x08 Unused * 0x10 32bit code * 0x18 32bit data/stack - * 0x20 16bit code - * 0x28 16bit data/stack */
gdt: @@ -103,13 +99,3 @@ gdt: .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) */