xtime.tv_usec update in timer interrupt

Hi,
     In update_wall_one_tick() function, we have following
           xtime.tv_usec += tick + time_adjust_step.

where tick = (1000000 + hz/2)/hz.

In systems where we have TSC why we cannot use that to update xtime.tv_usec. It
should give better time estimate. and the values that are retured in
sys_gettimeofday would also be more accurate .

Amol