[RFC PATCH v2 35/44] MdeModulePkg: Reserve a 16-bit protected mode code segment descriptor

["Lendacky, Thomas" <thomas.lendacky@amd.com>]

From: Tom Lendacky <thomas.lendacky@amd.com>

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2198

SEV-ES guest AP boot support needs to transition from 64-bit long mode
into 16-bit real mode. This will require a 16-bit code segment descriptor.
Reserve one of the spare segment descriptors (0x28) for this purpose.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Dandan Bi <dandan.bi@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
---
 MdeModulePkg/Core/DxeIplPeim/Ia32/DxeLoadFunc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/MdeModulePkg/Core/DxeIplPeim/Ia32/DxeLoadFunc.c b/MdeModulePkg/Core/DxeIplPeim/Ia32/DxeLoadFunc.c
index 630a3503f6ba..b9af22bede61 100644
--- a/MdeModulePkg/Core/DxeIplPeim/Ia32/DxeLoadFunc.c
+++ b/MdeModulePkg/Core/DxeIplPeim/Ia32/DxeLoadFunc.c
@@ -33,7 +33,7 @@ GLOBAL_REMOVE_IF_UNREFERENCED IA32_GDT gGdtEntries[] = {
 /* 0x10 */  {{0xffff, 0,  0,  0xf,  1,  0,  1,  0xf,  0,  0, 1,  1,  0}}, //linear code segment descriptor
 /* 0x18 */  {{0xffff, 0,  0,  0x3,  1,  0,  1,  0xf,  0,  0, 1,  1,  0}}, //system data segment descriptor
 /* 0x20 */  {{0xffff, 0,  0,  0xa,  1,  0,  1,  0xf,  0,  0, 1,  1,  0}}, //system code segment descriptor
-/* 0x28 */  {{0,      0,  0,  0,    0,  0,  0,  0,    0,  0, 0,  0,  0}}, //spare segment descriptor
+/* 0x28 */  {{0xffff, 0,  0,  0xa,  1,  0,  1,  0xf,  0,  0, 0,  1,  0}}, //system code16 segment descriptor
 /* 0x30 */  {{0xffff, 0,  0,  0x2,  1,  0,  1,  0xf,  0,  0, 1,  1,  0}}, //system data segment descriptor
 /* 0x38 */  {{0xffff, 0,  0,  0xa,  1,  0,  1,  0xf,  0,  1, 0,  1,  0}}, //system code segment descriptor
 /* 0x40 */  {{0,      0,  0,  0,    0,  0,  0,  0,    0,  0, 0,  0,  0}}, //spare segment descriptor
-- 
2.17.1