UefiCpuPkg: Add a 16-bit protected mode code segment descriptor

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>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/UefiCpuPkg/CpuDxe/CpuGdt.h b/UefiCpuPkg/CpuDxe/CpuGdt.h
index 3a0210b2f1..1c94487cbe 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)