diff --git a/OvmfPkg/Include/Library/XenHypercallLib.h b/OvmfPkg/Include/Library/XenHypercallLib.h
index 509855d302..36e3344e2f 100644
--- a/OvmfPkg/Include/Library/XenHypercallLib.h
+++ b/OvmfPkg/Include/Library/XenHypercallLib.h
@@ -58,6 +58,7 @@ XenHypercall2 (
   @return   The value of the asked parameter or 0 in case of error.
 **/
 UINT64
+EFIAPI
 XenHypercallHvmGetParam (
   UINT32 Index
   );
@@ -72,6 +73,7 @@ XenHypercallHvmGetParam (
            otherwise, an error code.
 **/
 INTN
+EFIAPI
 XenHypercallMemoryOp (
   IN     UINTN Operation,
   IN OUT VOID *Arguments
@@ -87,6 +89,7 @@ XenHypercallMemoryOp (
            otherwise, an error code.
 **/
 INTN
+EFIAPI
 XenHypercallEventChannelOp (
   IN     INTN Operation,
   IN OUT VOID *Arguments
diff --git a/OvmfPkg/Library/XenHypercallLib/XenHypercall.c b/OvmfPkg/Library/XenHypercallLib/XenHypercall.c
index 82cdbd9bdb..5ea5c45818 100644
--- a/OvmfPkg/Library/XenHypercallLib/XenHypercall.c
+++ b/OvmfPkg/Library/XenHypercallLib/XenHypercall.c
@@ -22,6 +22,7 @@
 #include <Library/XenHypercallLib.h>
 
 UINT64
+EFIAPI
 XenHypercallHvmGetParam (
   IN UINT32        Index
   )
@@ -43,6 +44,7 @@ XenHypercallHvmGetParam (
 }
 
 INTN
+EFIAPI
 XenHypercallMemoryOp (
   IN     UINTN Operation,
   IN OUT VOID *Arguments
@@ -53,6 +55,7 @@ XenHypercallMemoryOp (
 }
 
 INTN
+EFIAPI
 XenHypercallEventChannelOp (
   IN     INTN Operation,
   IN OUT VOID *Arguments