Ring3: Added SysCallReturnToCore.

MdeModulePkg/Core/Dxe/SysCall/X64/CoreBootServices.nasm

@@ -123,6 +123,10 @@ ASM_PFX(CoreBootServices):
     mov     fs, ax
     mov     gs, ax
 
+    ; Special case for SysCallReturnToCore.
+    cmp     r10, 0
+    je      coreReturnAddress
+
     ; Save User Stack pointers and switch to Core SysCall Stack.
     mov     rax, [ASM_PFX(gCoreSysCallStackTop)]
     sub     rax, 8
@@ -229,6 +233,8 @@ ASM_PFX(CallRing3):
 o64 sysret
 
 coreReturnAddress:
+    mov     rsp, [ASM_PFX(CoreRsp)]
+    mov     rbp, [ASM_PFX(CoreRbp)]
     ret
 
 SECTION .data

MdeModulePkg/Core/Dxe/SysCall/X64/Ring3Dxe.nasm

@@ -28,5 +28,7 @@ ASM_PFX(_ModuleEntryPoint):
     mov rdx, r9
 
     call r8
-    
-    ret
+
+    mov     r10, 0
+
+    syscall

MdePkg/Include/Uefi/UefiSpec.h

@@ -2014,6 +2014,7 @@ typedef struct {
 } EFI_BOOT_SERVICES;
 
 typedef enum {
+  SysCallReturnToCore                      = 0,
   SysCallLocateProtocol                    = 1,
   SysCallOpenProtocol                      = 2,
   SysCallInstallMultipleProtocolInterfaces = 3,