OvmfPkg: save on I/O port accesses when the debug port is not in use

When SEV is enabled, every debug message printed by OVMF to the
QEMU debug port traps from the guest to QEMU character by character
because "REP OUTSB" cannot be used by IoWriteFifo8.  Furthermore,
when OVMF is built with the DEBUG_VERBOSE bit (value 0x00400000)
enabled in "gEfiMdePkgTokenSpaceGuid.PcdDebugPrintErrorLevel", then the
OvmfPkg/IoMmuDxe driver, and the OvmfPkg/Library/BaseMemEncryptSevLib
library instance that is built into it, produce a huge amount of
log messages.  Therefore, in SEV guests, the boot time impact is huge
(about 45 seconds _additional_ time spent writing to the debug port).

While these messages are very useful for analyzing guest behavior,
most of the time the user won't be capturing the OVMF debug log.
In fact libvirt does not provide a method for configuring log capture;
users that wish to do this (or are instructed to do this) have to resort
to <qemu:arg>.

The debug console device provides a handy detection mechanism; when read,
it returns 0xE9 (which is very much unlike the 0xFF that is returned by
an unused port).  Use it to skip the possibly expensive OUT instructions
when the debug I/O port isn't plugged anywhere.

For SEC, the debug port has to be read before each full message.
However:

- if the debug port is available, then reading one byte before writing
a full message isn't tragic, especially because SEC doesn't print many
messages

- if the debug port is not available, then reading one byte instead of
writing a full message is still a win.

Contributed-under: TianoCore Contribution Agreement 1.0
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen (Intel address) <jordan.l.justen@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>

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 ();
+}