perf_event_mmap_page.time_zero

If cap_usr_time_zero, the hardware clock (e.g. TSC) can be calculated from sample timestamps.

time = timestamp - time_zero; quot = time / time_mult; rem = time % time_mult; cyc = (quot << time_shift) + (rem << time_shift) / time_mult;

And vice versa:

quot = cyc >> time_shift; rem = cyc & (((u64)1 << time_shift) - 1); timestamp = time_zero + quot * time_mult + ((rem * time_mult) >> time_shift);