diff --git a/StandaloneMmPkg/Drivers/StandaloneMmCpu/EventHandle.c b/StandaloneMmPkg/Drivers/StandaloneMmCpu/EventHandle.c
index 8cc1477e9e19..f482c0cef28e 100644
--- a/StandaloneMmPkg/Drivers/StandaloneMmCpu/EventHandle.c
+++ b/StandaloneMmPkg/Drivers/StandaloneMmCpu/EventHandle.c
@@ -117,6 +117,7 @@ CheckBufferAddr (
   @retval   EFI_UNSUPPORTED         Operation not supported.
 **/
 EFI_STATUS
+EFIAPI
 PiMmStandaloneMmCpuDriverEntry (
   IN UINTN  EventId,
   IN UINTN  CpuNumber,
diff --git a/StandaloneMmPkg/Drivers/StandaloneMmCpu/StandaloneMmCpu.c b/StandaloneMmPkg/Drivers/StandaloneMmCpu/StandaloneMmCpu.c
index c5ec1a5a80c5..70f2735e15d5 100644
--- a/StandaloneMmPkg/Drivers/StandaloneMmCpu/StandaloneMmCpu.c
+++ b/StandaloneMmPkg/Drivers/StandaloneMmCpu/StandaloneMmCpu.c
@@ -89,6 +89,7 @@ GetGuidedHobData (
                                   driver endpoint descriptor.
 **/
 EFI_STATUS
+EFIAPI
 StandaloneMmCpuInitialize (
   IN EFI_HANDLE           ImageHandle,   // not actual imagehandle
   IN EFI_MM_SYSTEM_TABLE  *SystemTable   // not actual systemtable
diff --git a/StandaloneMmPkg/Include/StandaloneMmCpu.h b/StandaloneMmPkg/Include/StandaloneMmCpu.h
index 1dce7c132ec2..4045ca62a7ab 100644
--- a/StandaloneMmPkg/Include/StandaloneMmCpu.h
+++ b/StandaloneMmPkg/Include/StandaloneMmCpu.h
@@ -17,6 +17,7 @@
 
 typedef
 EFI_STATUS
+EFIAPI
 (*PI_MM_CPU_DRIVER_ENTRYPOINT) (
   IN UINTN  EventId,
   IN UINTN  CpuNumber,
@@ -59,6 +60,7 @@ extern EFI_MM_CONFIGURATION_PROTOCOL  mMmConfig;
   @retval   EFI_OUT_OF_RESOURCES    Out of resources.
   @retval   EFI_UNSUPPORTED         Operation not supported.
 **/
+EFIAPI
 EFI_STATUS
 PiMmStandaloneMmCpuDriverEntry (
   IN UINTN  EventId,