4 files changed, 127 insertions, 9 deletions
diff --git a/OvmfPkg/Library/PlatformDebugLibIoPort/DebugLib.c b/OvmfPkg/Library/PlatformDebugLibIoPort/DebugLib.c
index 5a1c86f2c3..36cde54976 100644
--- a/OvmfPkg/Library/PlatformDebugLibIoPort/DebugLib.c
+++ b/OvmfPkg/Library/PlatformDebugLibIoPort/DebugLib.c
@@ -22,6 +22,7 @@
 #include <Library/PcdLib.h>
 #include <Library/BaseMemoryLib.h>
 #include <Library/DebugPrintErrorLevelLib.h>
+#include "DebugLibDetect.h"
 
 //
 // Define the maximum debug and assert message length that this library supports
@@ -61,9 +62,10 @@ DebugPrint (
   ASSERT (Format != NULL);
 
   //
-  // Check driver debug mask value and global mask
+  // Check if the global mask disables this message or the device is inactive
   //
-  if ((ErrorLevel & GetDebugPrintErrorLevel ()) == 0) {
+  if ((ErrorLevel & GetDebugPrintErrorLevel ()) == 0 ||
+      !PlatformDebugLibIoPortFound ()) {
     return;
   }
 
@@ -120,9 +122,11 @@ DebugAssert (
              FileName, (UINT64)LineNumber, Description);
 
   //
-  // Send the print string to the debug I/O port
+  // Send the print string to the debug I/O port, if present
   //
-  IoWriteFifo8 (PcdGet16 (PcdDebugIoPort), Length, Buffer);
+  if (PlatformDebugLibIoPortFound ()) {
+    IoWriteFifo8 (PcdGet16 (PcdDebugIoPort), Length, Buffer);
+  }
 
   //
   // Generate a Breakpoint, DeadLoop, or NOP based on PCD settings
@@ -265,3 +269,19 @@ DebugPrintLevelEnabled (
 {
   return (BOOLEAN) ((ErrorLevel & PcdGet32(PcdFixedDebugPrintErrorLevel)) != 0);
 }
+
+/**
+  Return the result of detecting the debug I/O port device.
+
+  @retval TRUE   if the debug I/O port device was detected.
+  @retval FALSE  otherwise
+
+**/
+BOOLEAN
+EFIAPI
+PlatformDebugLibIoPortDetect (
+  VOID
+  )
+{
+  return IoRead8 (PcdGet16 (PcdDebugIoPort)) == BOCHS_DEBUG_PORT_MAGIC;
+}
diff --git a/OvmfPkg/Library/PlatformDebugLibIoPort/DebugLibDetect.c b/OvmfPkg/Library/PlatformDebugLibIoPort/DebugLibDetect.c
index bad054f286..81c44eece9 100644
--- a/OvmfPkg/Library/PlatformDebugLibIoPort/DebugLibDetect.c
+++ b/OvmfPkg/Library/PlatformDebugLibIoPort/DebugLibDetect.c
@@ -1,6 +1,6 @@
 /** @file
-  Constructor code for QEMU debug port library.
-  Non-SEC instance.
+  Detection code for QEMU debug port.
+  Non-SEC instance, caches the result of detection.
 
   Copyright (c) 2017, Red Hat, Inc.<BR>
   This program and the accompanying materials
@@ -14,9 +14,16 @@
 **/
 
 #include <Base.h>
+#include "DebugLibDetect.h"
+
+//
+// Set to TRUE if the debug I/O port is enabled
+//
+STATIC BOOLEAN mDebugIoPortFound = FALSE;
 
 /**
-  This constructor function does not have anything to do.
+  This constructor function checks if the debug I/O port device is present,
+  caching the result for later use.
 
   @retval RETURN_SUCCESS   The constructor always returns RETURN_SUCCESS.
 
@@ -27,5 +34,22 @@ PlatformDebugLibIoPortConstructor (
   VOID
   )
 {
+  mDebugIoPortFound = PlatformDebugLibIoPortDetect();
   return RETURN_SUCCESS;
 }
+
+/**
+  Return the cached result of detecting the debug I/O port device.
+
+  @retval TRUE   if the debug I/O port device was detected.
+  @retval FALSE  otherwise
+
+**/
+BOOLEAN
+EFIAPI
+PlatformDebugLibIoPortFound (
+  VOID
+  )
+{
+  return mDebugIoPortFound;
+}
diff --git a/OvmfPkg/Library/PlatformDebugLibIoPort/DebugLibDetect.h b/OvmfPkg/Library/PlatformDebugLibIoPort/DebugLibDetect.h
new file mode 100644
index 0000000000..1f739b55d8
--- /dev/null
+++ b/OvmfPkg/Library/PlatformDebugLibIoPort/DebugLibDetect.h
@@ -0,0 +1,57 @@
+/** @file
+  Base Debug library instance for QEMU debug port.
+  It uses PrintLib to send debug messages to a fixed I/O port.
+
+  Copyright (c) 2017, Red Hat, Inc.<BR>
+  This program and the accompanying materials
+  are licensed and made available under the terms and conditions of the BSD License
+  which accompanies this distribution.  The full text of the license may be found at
+  http://opensource.org/licenses/bsd-license.php.
+
+  THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
+  WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
+
+**/
+
+#ifndef __DEBUG_IO_PORT_DETECT_H__
+#define __DEBUG_IO_PORT_DETECT_H__
+
+#include <Base.h>
+
+//
+// The constant value that is read from the debug I/O port
+//
+#define BOCHS_DEBUG_PORT_MAGIC    0xE9
+
+
+/**
+  Helper function to return whether the virtual machine has a debug I/O port.
+  PlatformDebugLibIoPortFound can call this function directly or cache the
+  result.
+
+  @retval TRUE   if the debug I/O port device was detected.
+  @retval FALSE  otherwise
+
+**/
+BOOLEAN
+EFIAPI
+PlatformDebugLibIoPortDetect (
+  VOID
+  );
+
+/**
+  Return whether the virtual machine has a debug I/O port.  DebugLib.c
+  calls this function instead of PlatformDebugLibIoPortDetect, to allow
+  caching if possible.
+
+  @retval TRUE   if the debug I/O port device was detected.
+  @retval FALSE  otherwise
+
+**/
+BOOLEAN
+EFIAPI
+PlatformDebugLibIoPortFound (
+  VOID
+  );
+
+#endif
diff --git a/OvmfPkg/Library/PlatformDebugLibIoPort/DebugLibDetectRom.c b/OvmfPkg/Library/PlatformDebugLibIoPort/DebugLibDetectRom.c
index 83a118a0f7..b950919675 100644
--- a/OvmfPkg/Library/PlatformDebugLibIoPort/DebugLibDetectRom.c
+++ b/OvmfPkg/Library/PlatformDebugLibIoPort/DebugLibDetectRom.c
@@ -1,6 +1,6 @@
 /** @file
-  Constructor code for QEMU debug port library.
-  SEC instance.
+  Detection code for QEMU debug port.
+  SEC instance, cannot cache the result of detection.
 
   Copyright (c) 2017, Red Hat, Inc.<BR>
   This program and the accompanying materials
@@ -14,6 +14,7 @@
 **/
 
 #include <Base.h>
+#include "DebugLibDetect.h"
 
 /**
   This constructor function does not have anything to do.
@@ -29,3 +30,19 @@ PlatformRomDebugLibIoPortConstructor (
 {
   return RETURN_SUCCESS;
 }
+
+/**
+  Return the result of detecting the debug I/O port device.
+
+  @retval TRUE   if the debug I/O port device was detected.
+  @retval FALSE  otherwise
+
+**/
+BOOLEAN
+EFIAPI
+PlatformDebugLibIoPortFound (
+  VOID
+  )
+{
+  return PlatformDebugLibIoPortDetect ();
+}