OvmfPkg/XenBusDxe/TestAndClearBit.c

/** @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;
}