1 files changed, 134 insertions, 0 deletions
diff --git a/OvmfPkg/XenResetVector/Ia16/Real16ToFlat32.asm b/OvmfPkg/XenResetVector/Ia16/Real16ToFlat32.asm
new file mode 100644
index 0000000000..5c329bfaea
--- /dev/null
+++ b/OvmfPkg/XenResetVector/Ia16/Real16ToFlat32.asm
@@ -0,0 +1,134 @@
+;------------------------------------------------------------------------------
+; @file
+; Transition from 16 bit real mode into 32 bit flat protected mode
+;
+; Copyright (c) 2008 - 2010, Intel Corporation. All rights reserved.<BR>
+; Copyright (c) 2019, Citrix Systems, Inc.
+; SPDX-License-Identifier: BSD-2-Clause-Patent
+;
+;------------------------------------------------------------------------------
+
+%define SEC_DEFAULT_CR0  0x00000023
+%define SEC_DEFAULT_CR4  0x640
+
+BITS    16
+
+;
+; Modified:  EAX, EBX
+;
+; @param[out]     DS       Selector allowing flat access to all addresses
+; @param[out]     ES       Selector allowing flat access to all addresses
+; @param[out]     FS       Selector allowing flat access to all addresses
+; @param[out]     GS       Selector allowing flat access to all addresses
+; @param[out]     SS       Selector allowing flat access to all addresses
+;
+TransitionFromReal16To32BitFlat:
+
+    debugShowPostCode POSTCODE_16BIT_MODE
+
+    cli
+
+    mov     bx, 0xf000
+    mov     ds, bx
+
+    mov     bx, ADDR16_OF(gdtr)
+
+o32 lgdt    [cs:bx]
+
+    mov     eax, SEC_DEFAULT_CR0
+    mov     cr0, eax
+
+    jmp     LINEAR_CODE_SEL:dword ADDR_OF(jumpTo32BitAndLandHere)
+BITS    32
+jumpTo32BitAndLandHere:
+
+    mov     eax, SEC_DEFAULT_CR4
+    mov     cr4, eax
+
+    debugShowPostCode POSTCODE_32BIT_MODE
+
+    mov     ax, LINEAR_SEL
+    mov     ds, ax
+    mov     es, ax
+    mov     fs, ax
+    mov     gs, ax
+    mov     ss, ax
+
+    OneTimeCallRet TransitionFromReal16To32BitFlat
+
+ALIGN   2
+
+gdtr:
+    dw      GDT_END - GDT_BASE - 1   ; GDT limit
+    dd      ADDR_OF(GDT_BASE)
+
+ALIGN   16
+
+;
+; Macros for GDT entries
+;
+
+%define  PRESENT_FLAG(p) (p << 7)
+%define  DPL(dpl) (dpl << 5)
+%define  SYSTEM_FLAG(s) (s << 4)
+%define  DESC_TYPE(t) (t)
+
+; Type: data, expand-up, writable, accessed
+%define  DATA32_TYPE 3
+
+; Type: execute, readable, expand-up, accessed
+%define  CODE32_TYPE 0xb
+
+; Type: execute, readable, expand-up, accessed
+%define  CODE64_TYPE 0xb
+
+%define  GRANULARITY_FLAG(g) (g << 7)
+%define  DEFAULT_SIZE32(d) (d << 6)
+%define  CODE64_FLAG(l) (l << 5)
+%define  UPPER_LIMIT(l) (l)
+
+;
+; The Global Descriptor Table (GDT)
+;
+
+GDT_BASE:
+; null descriptor
+NULL_SEL            equ $-GDT_BASE
+    DW      0            ; limit 15:0
+    DW      0            ; base 15:0
+    DB      0            ; base 23:16
+    DB      0            ; sys flag, dpl, type
+    DB      0            ; limit 19:16, flags
+    DB      0            ; base 31:24
+
+; linear data segment descriptor
+LINEAR_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(DATA32_TYPE)
+    DB      GRANULARITY_FLAG(1)|DEFAULT_SIZE32(1)|CODE64_FLAG(0)|UPPER_LIMIT(0xf)
+    DB      0            ; base 31:24
+
+; linear code segment descriptor
+LINEAR_CODE_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(1)|CODE64_FLAG(0)|UPPER_LIMIT(0xf)
+    DB      0            ; base 31:24
+
+%ifdef ARCH_X64
+; linear code (64-bit) segment descriptor
+LINEAR_CODE64_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(CODE64_TYPE)
+    DB      GRANULARITY_FLAG(1)|DEFAULT_SIZE32(0)|CODE64_FLAG(1)|UPPER_LIMIT(0xf)
+    DB      0            ; base 31:24
+%endif
+
+GDT_END:
+