1 files changed, 40 insertions, 0 deletions
diff --git a/roms/edk2/OvmfPkg/XenBusDxe/TestAndClearBit.c b/roms/edk2/OvmfPkg/XenBusDxe/TestAndClearBit.c
new file mode 100644
index 000000000..989ab7035
--- /dev/null
+++ b/roms/edk2/OvmfPkg/XenBusDxe/TestAndClearBit.c
@@ -0,0 +1,40 @@
+/** @file
+  Implementation of TestAndClearBit using compare-exchange primitive
+
+  Copyright (C) 2015, Linaro Ltd.
+  Copyright (c) 2015, Intel Corporation. All rights reserved.<BR>
+
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+
+**/
+
+#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 = (VOID*)((UINT8*) 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;
+}