OvmfPkg/XenResetVector: Add new entry point for Xen PVH

Add a new entry point for Xen PVH that enter directly in 32bits.

Information on the expected state of the machine when this entry point
is used can be found at:
https://xenbits.xenproject.org/docs/unstable/misc/pvh.html

Also, compare to the original file [1], the two `nop' of the "resetVector"
entry point are removed. There were introduced by 8332983e2e33
("UefiCpuPkg: Replace the un-necessary WBINVD instruction at the reset
vector with two NOPs in VTF0.", 2011-08-04), but don't seems to be
useful. This is the entry point used by HVM guest (hvmloader).

[1] UefiCpuPkg/ResetVector/Vtf0/Ia16/ResetVectorVtf0.asm

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1689
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20190813113119.14804-7-anthony.perard@citrix.com>

OvmfPkg/XenResetVector/Ia16/ResetVectorVtf0.asm

@@ -0,0 +1,79 @@
+;------------------------------------------------------------------------------
+; @file
+; First code executed by processor after resetting.
+;
+; Copyright (c) 2008 - 2014, Intel Corporation. All rights reserved.<BR>
+; Copyright (c) 2019, Citrix Systems, Inc.
+;
+; SPDX-License-Identifier: BSD-2-Clause-Patent
+;
+;------------------------------------------------------------------------------
+
+BITS    16
+
+ALIGN   16
+
+;
+; Pad the image size to 4k when page tables are in VTF0
+;
+; If the VTF0 image has page tables built in, then we need to make
+; sure the end of VTF0 is 4k above where the page tables end.
+;
+; This is required so the page tables will be 4k aligned when VTF0 is
+; located just below 0x100000000 (4GB) in the firmware device.
+;
+%ifdef ALIGN_TOP_TO_4K_FOR_PAGING
+    TIMES (0x1000 - ($ - EndOfPageTables) - (fourGigabytes - xenPVHEntryPoint)) DB 0
+%endif
+
+BITS    32
+xenPVHEntryPoint:
+;
+; Entry point to use when running as a Xen PVH guest. (0xffffffd0)
+;
+; Description of the expected state of the machine when this entry point is
+; used can be found at:
+; https://xenbits.xenproject.org/docs/unstable/misc/pvh.html
+;
+    jmp     xenPVHMain
+
+BITS    16
+ALIGN   16
+
+applicationProcessorEntryPoint:
+;
+; Application Processors entry point
+;
+; GenFv generates code aligned on a 4k boundary which will jump to this
+; location.  (0xffffffe0)  This allows the Local APIC Startup IPI to be
+; used to wake up the application processors.
+;
+    jmp     EarlyApInitReal16
+
+ALIGN   8
+
+    DD      0
+
+;
+; The VTF signature
+;
+; VTF-0 means that the VTF (Volume Top File) code does not require
+; any fixups.
+;
+vtfSignature:
+    DB      'V', 'T', 'F', 0
+
+ALIGN   16
+
+resetVector:
+;
+; Reset Vector
+;
+; This is where the processor will begin execution
+;
+    jmp     EarlyBspInitReal16
+
+ALIGN   16
+
+fourGigabytes:
+

OvmfPkg/XenResetVector/Ia32/XenPVHMain.asm

@@ -0,0 +1,49 @@
+;------------------------------------------------------------------------------
+; @file
+; An entry point use by Xen when a guest is started in PVH mode.
+;
+; Copyright (c) 2019, Citrix Systems, Inc.
+;
+; SPDX-License-Identifier: BSD-2-Clause-Patent
+;
+;------------------------------------------------------------------------------
+
+BITS    32
+
+xenPVHMain:
+    ;
+    ; 'BP' to indicate boot-strap processor
+    ;
+    mov     di, 'BP'
+
+    ;
+    ; ESP will be used as initial value of the EAX register
+    ; in Main.asm
+    ;
+    xor     esp, esp
+
+    mov     ebx, ADDR_OF(gdtr)
+    lgdt    [ebx]
+
+    mov     eax, SEC_DEFAULT_CR0
+    mov     cr0, eax
+
+    jmp     LINEAR_CODE_SEL:ADDR_OF(.jmpToNewCodeSeg)
+.jmpToNewCodeSeg:
+
+    mov     eax, SEC_DEFAULT_CR4
+    mov     cr4, eax
+
+    mov     ax, LINEAR_SEL
+    mov     ds, ax
+    mov     es, ax
+    mov     fs, ax
+    mov     gs, ax
+    mov     ss, ax
+
+    ;
+    ; Jump to the main routine of the pre-SEC code
+    ; skiping the 16-bit part of the routine and
+    ; into the 32-bit flat mode part
+    ;
+    OneTimeCallRet TransitionFromReal16To32BitFlat

OvmfPkg/XenResetVector/XenResetVector.nasmb

@@ -63,6 +63,7 @@
 %include "Ia16/Init16.asm"
 
 %include "Main.asm"
+%include "Ia32/XenPVHMain.asm"
 
 %include "Ia16/ResetVectorVtf0.asm"