[RFC PATCH v3 34/43] UefiCpuPkg: Add a 16-bit protected mode code segment descriptor

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

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

A hypervisor is not allowed to update an SEV-ES guests register state,
so when booting an SEV-ES guest AP, the hypervisor is not allowed to
set the RIP to the guest requested value. Instead, an SEV-ES AP must be
transition from 64-bit long mode to 16-bit real mode in response to an
INIT-SIPI-SIPI sequence. This requires a 16-bit code segment descriptor.
For PEI, create this descriptor in the reset vector GDT table. For DXE,
create this descriptor from the newly reserved entry at location 0x28.

Cc: Eric Dong <eric.dong@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
---
 UefiCpuPkg/CpuDxe/CpuGdt.h                          | 4 ++--
 UefiCpuPkg/CpuDxe/CpuGdt.c                          | 8 ++++----
 UefiCpuPkg/ResetVector/Vtf0/Ia16/Real16ToFlat32.asm | 9 +++++++++
 3 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/UefiCpuPkg/CpuDxe/CpuGdt.h b/UefiCpuPkg/CpuDxe/CpuGdt.h
index e5c36f37b96a..80e224b47fcd 100644
--- a/UefiCpuPkg/CpuDxe/CpuGdt.h
+++ b/UefiCpuPkg/CpuDxe/CpuGdt.h
@@ -36,7 +36,7 @@ struct _GDT_ENTRIES {
   GDT_ENTRY LinearCode;
   GDT_ENTRY SysData;
   GDT_ENTRY SysCode;
-  GDT_ENTRY Spare4;
+  GDT_ENTRY SysCode16;
   GDT_ENTRY LinearData64;
   GDT_ENTRY LinearCode64;
   GDT_ENTRY Spare5;
@@ -49,7 +49,7 @@ struct _GDT_ENTRIES {
 #define LINEAR_CODE_SEL   OFFSET_OF (GDT_ENTRIES, LinearCode)
 #define SYS_DATA_SEL      OFFSET_OF (GDT_ENTRIES, SysData)
 #define SYS_CODE_SEL      OFFSET_OF (GDT_ENTRIES, SysCode)
-#define SPARE4_SEL        OFFSET_OF (GDT_ENTRIES, Spare4)
+#define SYS_CODE16_SEL    OFFSET_OF (GDT_ENTRIES, SysCode16)
 #define LINEAR_DATA64_SEL OFFSET_OF (GDT_ENTRIES, LinearData64)
 #define LINEAR_CODE64_SEL OFFSET_OF (GDT_ENTRIES, LinearCode64)
 #define SPARE5_SEL        OFFSET_OF (GDT_ENTRIES, Spare5)
diff --git a/UefiCpuPkg/CpuDxe/CpuGdt.c b/UefiCpuPkg/CpuDxe/CpuGdt.c
index 87fd6955f24b..6a80829be884 100644
--- a/UefiCpuPkg/CpuDxe/CpuGdt.c
+++ b/UefiCpuPkg/CpuDxe/CpuGdt.c
@@ -70,14 +70,14 @@ STATIC GDT_ENTRIES GdtTemplate = {
     0x0,
   },
   //
-  // SPARE4_SEL
+  // SYS_CODE16_SEL
   //
   {
-    0x0,            // limit 15:0
+    0x0FFFF,        // limit 15:0
     0x0,            // base 15:0
     0x0,            // base 23:16
-    0x0,            // type
-    0x0,            // limit 19:16, flags
+    0x09A,          // present, ring 0, code, execute/read
+    0x08F,          // page-granular, 16-bit
     0x0,            // base 31:24
   },
   //
diff --git a/UefiCpuPkg/ResetVector/Vtf0/Ia16/Real16ToFlat32.asm b/UefiCpuPkg/ResetVector/Vtf0/Ia16/Real16ToFlat32.asm
index ce4ebfffb688..0e79a3984b16 100644
--- a/UefiCpuPkg/ResetVector/Vtf0/Ia16/Real16ToFlat32.asm
+++ b/UefiCpuPkg/ResetVector/Vtf0/Ia16/Real16ToFlat32.asm
@@ -129,5 +129,14 @@ LINEAR_CODE64_SEL   equ $-GDT_BASE
     DB      0            ; base 31:24
 %endif
 
+; linear code segment descriptor
+LINEAR_CODE16_SEL     equ $-GDT_BASE
+    DW      0xffff       ; limit 15:0
+    DW      0            ; base 15:0
+    DB      0            ; base 23:16
+    DB      PRESENT_FLAG(1)|DPL(0)|SYSTEM_FLAG(1)|DESC_TYPE(CODE32_TYPE)
+    DB      GRANULARITY_FLAG(1)|DEFAULT_SIZE32(0)|CODE64_FLAG(0)|UPPER_LIMIT(0xf)
+    DB      0            ; base 31:24
+
 GDT_END:
 
-- 
2.17.1