[HarmonyOS] 鸿蒙轻内核虚拟内存基础知识：虚拟内存进程空间编号

摘要：本文介绍了OpenHarmony鸿蒙轻内核LiteOS-A的虚拟地址空间编号知识，详细分析进程空间编号的申请与释放操作。

#define MMU_ARM_ASID_BITS       8
......
STATIC UINTPTR g_asidPool[BITMAP_NUM_WORDS(1UL << MMU_ARM_ASID_BITS)];

status_t OsAllocAsid(UINT32 *asid)
{
  UINT32 flags;
  LOS_SpinLockSave(&g_cpuAsidLock, &flags);
⑴  UINT32 firstZeroBit = LOS_BitmapFfz(g_asidPool, 1UL << MMU_ARM_ASID_BITS);
⑵  if (firstZeroBit >= 0 && firstZeroBit < (1UL << MMU_ARM_ASID_BITS)) {
⑶    LOS_BitmapSetNBits(g_asidPool, firstZeroBit, 1);
⑷    *asid = firstZeroBit;
    LOS_SpinUnlockRestore(&g_cpuAsidLock, flags);
    return LOS_OK;
  }
  LOS_SpinUnlockRestore(&g_cpuAsidLock, flags);
  return firstZeroBit;
}

VOID OsFreeAsid(UINT32 asid)
{
  UINT32 flags;
  LOS_SpinLockSave(&g_cpuAsidLock, &flags);
  LOS_BitmapClrNBits(g_asidPool, asid, 1);
  LOS_SpinUnlockRestore(&g_cpuAsidLock, flags);
}