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arch/x86: Unify GDT entries

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Currently there are 3 GDTs (Global Descriptor Tables) being used on x86:
- preRAM (gdt_init.S)
- SMM (smm_stub.S)
- RAM (c_start.S)

They have different layouts and thus different offsets for the segments
being used in assembly code. Stop using different GDT segments and
ensure that for ROM (preRAM + SMM) and RAM (ramstage) the segments
match. RAM will have additional entries, not found in pre RAM GDT,
but the segments for protected mode and 64-bit mode now match in
all stages.

This allows to use the same defines in all stages. It also drops the
need to know in which stage the code is compiled and it's no longer
necessary to switch the code segment between stages.

While at it fix the comments in the ramstage GDT and drop unused
declarations from header files, always set the accessed bit and drop
GDT_CODE_ACPI_SEG.

Change-Id: I208496e6e4cc82833636f4f42503b44b0d702b9e
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/87255
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Shuo Liu <shuo.liu@intel.com>
Reviewed-by: Maximilian Brune <maximilian.brune@9elements.com>
This commit is contained in:
Patrick Rudolph 2025-04-10 08:51:55 +02:00 • committed by Matt DeVillier
commit a6be271e63
14 changed files with 89 additions and 135 deletions
Download patch file Download diff file Expand all files Collapse all files

30
src/device/oprom/realmode/x86_asm.S
View file

@ -3,7 +3,7 @@
#define REALMODE_BASE 0x600
#define RELOCATED(x) (x - __realmode_code + REALMODE_BASE)
#include <arch/ram_segs.h>
#include <cpu/x86/gdt.h>
/* CR0 bits */
#define PE (1 << 0)
@ -97,7 +97,7 @@ __realmode_call:
movl %eax, __registers + 20 /* edi */
/* Activate the right segment descriptor real mode. */
ljmp $RAM_CODE16_SEG, $RELOCATED(1f)
ljmp $GDT_CODE16_SEG, $RELOCATED(1f)
1:
.code16
/* 16 bit code from here on... */
@ -107,7 +107,7 @@ __realmode_call:
* configurations (limits, writability, etc.) once
* protected mode is turned off.
*/
mov $RAM_DATA16_SEG, %ax
mov $GDT_DATA16_SEG, %ax
mov %ax, %ds
mov %ax, %es
mov %ax, %fs
@ -144,13 +144,13 @@ __realmode_call:
movl __registers + 16, %esi
movl __registers + 20, %edi
/* Set all segments to 0x0000, ds to 0x0040 */
/* Set all segments to 0x0000, ds to 0x0038 */
push %ax
xor %ax, %ax
mov %ax, %es
mov %ax, %fs
mov %ax, %gs
mov $RAM_DATA_ACPI_SEG, %ax
mov $GDT_DATA_ACPI_SEG, %ax
mov %ax, %ds
pop %ax
@ -177,10 +177,10 @@ __lcall_instr = RELOCATED(.)
/* Now that we are in protected mode
* jump to a 32 bit code segment.
*/
ljmpl $RAM_CODE_SEG, $RELOCATED(1f)
ljmpl $GDT_CODE_SEG, $RELOCATED(1f)
1:
.code32
mov $RAM_DATA_SEG, %ax
mov $GDT_DATA_SEG, %ax
mov %ax, %ds
mov %ax, %es
mov %ax, %fs
@ -233,7 +233,7 @@ __realmode_interrupt:
movl %eax, __registers + 20 /* edi */
/* This configures CS properly for real mode. */
ljmp $RAM_CODE16_SEG, $RELOCATED(1f)
ljmp $GDT_CODE16_SEG, $RELOCATED(1f)
1:
.code16 /* 16 bit code from here on... */
@ -241,7 +241,7 @@ __realmode_interrupt:
* descriptors. They will retain these configurations (limits,
* writability, etc.) once protected mode is turned off.
*/
mov $RAM_DATA16_SEG, %ax
mov $GDT_DATA16_SEG, %ax
mov %ax, %ds
mov %ax, %es
mov %ax, %fs
@ -305,10 +305,10 @@ __intXX_instr = RELOCATED(.)
movl %eax, %cr0
/* Now that we are in protected mode jump to a 32-bit code segment. */
ljmpl $RAM_CODE_SEG, $RELOCATED(1f)
ljmpl $GDT_CODE_SEG, $RELOCATED(1f)
1:
.code32
mov $RAM_DATA_SEG, %ax
mov $GDT_DATA_SEG, %ax
mov %ax, %ds
mov %ax, %es
mov %ax, %fs
@ -354,10 +354,10 @@ __interrupt_handler_16bit = RELOCATED(.)
movl %eax, %cr0
/* ... and jump to a 32 bit code segment. */
ljmpl $RAM_CODE_SEG, $RELOCATED(1f)
ljmpl $GDT_CODE_SEG, $RELOCATED(1f)
1:
.code32
mov $RAM_DATA_SEG, %ax
mov $GDT_DATA_SEG, %ax
mov %ax, %ds
mov %ax, %es
mov %ax, %fs
@ -371,14 +371,14 @@ __interrupt_handler_16bit = RELOCATED(.)
call *%eax
/* Now return to real mode ... */
ljmp $RAM_CODE16_SEG, $RELOCATED(1f)
ljmp $GDT_CODE16_SEG, $RELOCATED(1f)
1:
.code16
/* Load the segment registers with properly configured segment
* descriptors. They will retain these configurations (limits,
* writability, etc.) once protected mode is turned off.
*/
mov $RAM_DATA16_SEG, %ax
mov $GDT_DATA16_SEG, %ax
mov %ax, %ds
mov %ax, %es
mov %ax, %fs