OvmfPkg/QemuVideoDxe: Helper functions for unaligned port I/O.

The VMWare SVGA display device implemented by Qemu (-vga vmware) uses
an I/O-type BAR which is laid out such that some register offsets are
not aligned to the read/write width with which they are expected to be
accessed. (The register value port has an offset of 1 and requires
32 bit wide read/write access.)

The EFI_PCI_IO_PROTOCOL's Io.Read/Io.Write functions do not support
such unaligned I/O.

Before a driver for this device can be added to QemuVideoDxe, helper
functions for unaligned I/O are therefore required. This adds the
functions UnalignedIoWrite32 and UnalignedIoRead32, based on IoLib's
IoWrite32 and IoRead32, for the Ia32 and X64 architectures. Port I/O
requires inline assembly, so implementations are provided for the GCC,
ICC, and Microsoft compiler families. Such I/O is not possible on other
architectures, a dummy (ASSERT()ing) implementation is therefore
provided to satisfy the linker.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Suggested-by: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Phil Dennis-Jordan <phil@philjordan.eu>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

OvmfPkg/QemuVideoDxe/UnalignedIoUnsupported.c

@@ -0,0 +1,66 @@
+/** @file
+  Unaligned port I/O dummy implementation for platforms which do not support it.
+
+  Copyright (c) 2017, Phil Dennis-Jordan.<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.
+
+**/
+
+
+#include <Library/DebugLib.h>
+#include "UnalignedIoInternal.h"
+
+/**
+  Performs a 32-bit write to the specified, possibly unaligned I/O-type
+  address.
+
+  Writes the 32-bit I/O port specified by Port with the value specified by
+  Value and returns Value. This function must guarantee that all I/O read and
+  write operations are serialized.
+
+  If 32-bit unaligned I/O port operations are not supported, then ASSERT().
+
+  @param[in]  Port   I/O port address
+  @param[in]  Value  32-bit word to write
+
+  @return The value written to the I/O port.
+
+**/
+UINT32
+UnalignedIoWrite32 (
+  IN      UINTN                     Port,
+  IN      UINT32                    Value
+  )
+{
+  ASSERT (FALSE);
+  return Value;
+}
+
+/**
+  Reads a 32-bit word from the specified, possibly unaligned I/O-type address.
+
+  Reads the 32-bit I/O port specified by Port. The 32-bit read value is
+  returned. This function must guarantee that all I/O read and write operations
+  are serialized.
+
+  If 32-bit unaligned I/O port operations are not supported, then ASSERT().
+
+  @param[in]  Port  The I/O port to read.
+
+  @return The value read.
+
+**/
+UINT32
+UnalignedIoRead32 (
+  IN      UINTN                     Port
+  )
+{
+  ASSERT (FALSE);
+  return 0;
+}