typedef struct _ETW_REG_ENTRY
{
     LIST_ENTRY RegList;
     PETW_GUID_ENTRY GuidEntry;
     WORD Index;
     WORD Flags;
     UCHAR EnableMask;
     union
     {
          PETW_REPLY_QUEUE ReplyQueue;
          PETW_REG_ENTRY ReplySlot[4];
     };
     union
     {
          PEPROCESS Process;
          PVOID Callback;
     };
     PVOID CallbackContext;
} ETW_REG_ENTRY, *PETW_REG_ENTRY;

Windows Vista Kernel Structures