arch/x86: Relocate GDT in verstage, romstage, and postcar

In each stage keep GDT in the code region. This accommodates
platforms, such as glk, that are executing out of CAR. The
gdt is small and loading it is trivial so just do it unconditionally
instead of introducing another Kconfig.

BUG=b:78656686

Change-Id: I01ded6e9b358b23e04d92bef5263bfe8c2a5ec5a
Signed-off-by: Hannah Williams <hannah.williams@intel.com>
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/25895
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Justin TerAvest <teravest@chromium.org>

4 files changed, 58 insertions, 4 deletions

diff --git a/src/arch/x86/Makefile.inc b/src/arch/x86/Makefile.inc
index 0f4de166cb..df054f8748 100644
--- a/src/arch/x86/Makefile.inc
+++ b/src/arch/x86/Makefile.inc
@@ -171,6 +171,7 @@ endif # CONFIG_ARCH_BOOTBLOCK_X86_32 / CONFIG_ARCH_BOOTBLOCK_X86_64
 ifeq ($(CONFIG_ARCH_VERSTAGE_X86_32)$(CONFIG_ARCH_VERSTAGE_X86_64),y)
 
 verstage-y += boot.c
+verstage-$(CONFIG_VBOOT_SEPARATE_VERSTAGE) += gdt_init.S
 verstage-$(CONFIG_IDT_IN_EVERY_STAGE) += exception.c
 verstage-$(CONFIG_IDT_IN_EVERY_STAGE) += idt.S
 
@@ -206,6 +207,9 @@ ifeq ($(CONFIG_ARCH_ROMSTAGE_X86_32)$(CONFIG_ARCH_ROMSTAGE_X86_64),y)
 
 romstage-$(CONFIG_HAVE_ACPI_RESUME) += acpi_s3.c
 romstage-y += boot.c
+# gdt_init.S is included by entry32.inc when romstage is the first C
+# environment.
+romstage-$(CONFIG_C_ENVIRONMENT_BOOTBLOCK) += gdt_init.S
 romstage-y += cbmem.c
 romstage-y += cbfs_and_run.c
 romstage-$(CONFIG_ARCH_RAMSTAGE_X86_32) += cpu_common.c
@@ -283,6 +287,7 @@ postcar-generic-ccopts += -D__POSTCAR__
 
 postcar-$(CONFIG_HAVE_ACPI_RESUME) += acpi_s3.c
 postcar-y += boot.c
+postcar-y += gdt_init.S
 postcar-y += cbfs_and_run.c
 postcar-y += cbmem.c
 postcar-y += cpu_common.c
diff --git a/src/arch/x86/assembly_entry.S b/src/arch/x86/assembly_entry.S
index a5399b74a5..02f492cfaa 100644
--- a/src/arch/x86/assembly_entry.S
+++ b/src/arch/x86/assembly_entry.S
@@ -20,15 +20,18 @@
 
 /*
  * This path is for stages that are post bootblock when employing
- * CONFIG_C_ENVIRONMENT_BOOTBLOCK. There's no need to re-load the gdt,
- * etc as all those settings are cached within the processor. In order
- * to continue with C code execution one needs to set stack pointer and
- * clear CAR_GLOBAL variables that are stage specific.
+ * CONFIG_C_ENVIRONMENT_BOOTBLOCK. The gdt is reloaded to accommodate
+ * platforms that are executing out of CAR. In order to continue with
+ * C code execution one needs to set stack pointer and clear CAR_GLOBAL
+ * variables that are stage specific.
  */
 .section ".text._start", "ax", @progbits
 .global _start
 _start:
 
+	/* Migrate GDT to this text segment */
+	call	gdt_init
+
 	/* reset stack pointer to CAR stack */
 	mov	$_car_stack_end, %esp
 
diff --git a/src/arch/x86/exit_car.S b/src/arch/x86/exit_car.S
index 86d46ca95b..735399b8b1 100644
--- a/src/arch/x86/exit_car.S
+++ b/src/arch/x86/exit_car.S
@@ -30,6 +30,9 @@ _start:
 	/* Assume stack alignment doesn't matter here as chipset_teardown_car
 	   is expected to be implemented in assembly. */
 
+	/* Migrate GDT to this text segment */
+	call	gdt_init
+
 	/* chipset_teardown_car() is expected to disable cache-as-ram. */
 	call	chipset_teardown_car
 
diff --git a/src/arch/x86/gdt_init.S b/src/arch/x86/gdt_init.S
new file mode 100644
index 0000000000..6aa2a79b48
--- /dev/null
+++ b/src/arch/x86/gdt_init.S
@@ -0,0 +1,43 @@
+/*
+ * This file is part of the coreboot project.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; version 2 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+.code32
+.section ".text._gdt_", "ax", @progbits
+
+	.globl gdt_init
+gdt_init:
+	lgdt	%cs:gdtptr
+	ret
+
+.previous
+	.align	4
+.globl gdtptr
+gdt:
+gdtptr:
+	.word	gdt_end - gdt -1 /* compute the table limit */
+	.long	gdt		 /* we know the offset */
+	.word	0
+
+	/* selgdt 0x08, flat code segment */
+	.word	0xffff, 0x0000
+	.byte	0x00, 0x9b, 0xcf, 0x00 /* G=1 and 0x0f, So we get 4Gbytes
+					  for limit */
+
+	/* selgdt 0x10,flat data segment */
+	.word	0xffff, 0x0000
+	.byte	0x00, 0x93, 0xcf, 0x00
+
+	/* selgdt 0x18, flat code segment (64-bit) */
+	.word   0xffff, 0x0000
+	.byte   0x00, 0x9b, 0xaf, 0x00
+
+gdt_end: