[PATCH v15 40/46] UefiCpuPkg: Add a 16-bit protected mode code segment descriptor


Lendacky, Thomas
 

From: Tom Lendacky <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>
Reviewed-by: Eric Dong <eric.dong@intel.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 3a0210b2f172..1c94487cbee8 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 64efadeba601..a1ab543f2da5 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:

--
2.28.0

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