Ovmf/Xen: port XenBusDxe to other architectures

This patch updates XenBusDxe to use the 16-bit compare and exchange
function that was introduced for this purpose to the
BaseSynchronizationLib. It also provides a new generic implementation
of TestAndClearBit () using the same 16-bit compare and exchange, making
this module fully architecture agnostic.

Contributed-under: TianoCore Contribution Agreement 1.0
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16975 6f19259b-4bc3-4df7-8a09-765794883524

OvmfPkg/XenBusDxe/TestAndClearBit.c

@@ -0,0 +1,45 @@
+/** @file
+  Implementation of TestAndClearBit using compare-exchange primitive
+
+  Copyright (C) 2015, Linaro Ltd.
+
+  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 <Base.h>
+#include <Library/SynchronizationLib.h>
+
+INT32
+EFIAPI
+TestAndClearBit (
+  IN INT32            Bit,
+  IN VOID             *Address
+  )
+{
+  UINT16    Word, Read;
+  UINT16    Mask;
+
+  //
+  // Calculate the effective address relative to 'Address' based on the
+  // higher order bits of 'Bit'. Use signed shift instead of division to
+  // ensure we round towards -Inf, and end up with a positive shift in
+  // 'Bit', even if 'Bit' itself is negative.
+  //
+  Address += (Bit >> 4) * sizeof(UINT16);
+  Mask = 1U << (Bit & 15);
+
+  for (Word = *(UINT16 *) Address; Word & Mask; Word = Read) {
+    Read = InterlockedCompareExchange16 (Address, Word, Word & ~Mask);
+    if (Read == Word) {
+      return 1;
+    }
+  }
+  return 0;
+}