diff --git a/OvmfPkg/TdxDxe/X64/ApRunLoop.nasm b/OvmfPkg/TdxDxe/X64/ApRunLoop.nasm
index 0bef89c485..57560015f4 100644
--- a/OvmfPkg/TdxDxe/X64/ApRunLoop.nasm
+++ b/OvmfPkg/TdxDxe/X64/ApRunLoop.nasm
@@ -20,7 +20,7 @@ SECTION .text
 
 BITS 64
 
-%define TDVMCALL_EXPOSE_REGS_MASK       0xffec
+%define TDVMCALL_EXPOSE_REGS_MASK       0xffcc
 %define TDVMCALL                        0x0
 %define EXIT_REASON_CPUID               0xa
 
@@ -28,6 +28,30 @@ BITS 64
   db  0x66, 0x0f, 0x01, 0xcc
 %endmacro
 
+%macro tdcall_regs_preamble 2
+    mov rax, %1
+
+    xor rcx, rcx
+    mov ecx, %2
+
+    ; R10 = 0 (standard TDVMCALL)
+
+    xor r10d, r10d
+
+    ; Zero out unused (for standard TDVMCALL) registers to avoid leaking
+    ; secrets to the VMM.
+
+    xor esi, esi
+    xor edi, edi
+
+    xor edx, edx
+    xor ebp, ebp
+    xor r8d, r8d
+    xor r9d, r9d
+    xor r14, r14
+    xor r15, r15
+%endmacro
+
 ;
 ; Relocated Ap Mailbox loop
 ;
@@ -40,11 +64,9 @@ global ASM_PFX(AsmRelocateApMailBoxLoop)
 ASM_PFX(AsmRelocateApMailBoxLoop):
 AsmRelocateApMailBoxLoopStart:
 
-    mov         rax, TDVMCALL
-    mov         rcx, TDVMCALL_EXPOSE_REGS_MASK
-    xor         r10, r10
     mov         r11, EXIT_REASON_CPUID
     mov         r12, 0xb
+    tdcall_regs_preamble TDVMCALL, TDVMCALL_EXPOSE_REGS_MASK
     tdcall
     test        r10, r10
     jnz         Panic