typedef struct _PERFINFO_TRACE_HEADER
{
     union
     {
          ULONG Marker;
          struct
          {
               WORD Version;
               UCHAR HeaderType;
               UCHAR Flags;
          };
     };
     union
     {
          ULONG Header;
          WMI_TRACE_PACKET Packet;
     };
     union
     {
          UINT64 TS;
          LARGE_INTEGER SystemTime;
     };
     UCHAR Data[1];
} PERFINFO_TRACE_HEADER, *PPERFINFO_TRACE_HEADER;

Windows Vista Kernel Structures