diff --git a/OvmfPkg/Library/PlatformDebugLibIoPort/DebugLibDetect.c b/OvmfPkg/Library/PlatformDebugLibIoPort/DebugLibDetect.c
index 81c44eece9..e24cc834c2 100644
--- a/OvmfPkg/Library/PlatformDebugLibIoPort/DebugLibDetect.c
+++ b/OvmfPkg/Library/PlatformDebugLibIoPort/DebugLibDetect.c
@@ -16,14 +16,26 @@
 #include <Base.h>
 #include "DebugLibDetect.h"
 
+//
+// Set to TRUE if the debug I/O port has been checked
+//
+STATIC BOOLEAN mDebugIoPortChecked = FALSE;
+
 //
 // Set to TRUE if the debug I/O port is enabled
 //
 STATIC BOOLEAN mDebugIoPortFound = FALSE;
 
 /**
-  This constructor function checks if the debug I/O port device is present,
-  caching the result for later use.
+  This constructor function must not do anything.
+
+  Some modules consuming this library instance, such as the DXE Core, invoke
+  the DEBUG() macro before they explicitly call
+  ProcessLibraryConstructorList(). Therefore the auto-generated call from
+  ProcessLibraryConstructorList() to this constructor function may be preceded
+  by some calls to PlatformDebugLibIoPortFound() below. Hence
+  PlatformDebugLibIoPortFound() must not rely on anything this constructor
+  could set up.
 
   @retval RETURN_SUCCESS   The constructor always returns RETURN_SUCCESS.
 
@@ -34,12 +46,12 @@ PlatformDebugLibIoPortConstructor (
   VOID
   )
 {
-  mDebugIoPortFound = PlatformDebugLibIoPortDetect();
   return RETURN_SUCCESS;
 }
 
 /**
-  Return the cached result of detecting the debug I/O port device.
+  At the first call, check if the debug I/O port device is present, and cache
+  the result for later use. At subsequent calls, return the cached result.
 
   @retval TRUE   if the debug I/O port device was detected.
   @retval FALSE  otherwise
@@ -51,5 +63,9 @@ PlatformDebugLibIoPortFound (
   VOID
   )
 {
+  if (!mDebugIoPortChecked) {
+    mDebugIoPortFound = PlatformDebugLibIoPortDetect ();
+    mDebugIoPortChecked = TRUE;
+  }
   return mDebugIoPortFound;
 }