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This routine returns a monotonic count of the number of nanoseconds elapsed since the previous call. On arm64 it uses the generic system timer. The implementation multiplies the counter value by 10**9 then divides by the counter frequency, but this multiplication can overflow. This can result in trace records with non-monotonic timestamps, which breaks libdtrace's temporal ordering algorithm. An easy fix is to reverse the order of operations, since the counter frequency will in general be smaller than 10**9. (In fact, it's mandated to be 1Ghz in ARMv9, which makes life simple.) However, this can give a fair bit of error. Adopt the calculation used on amd64, with tweaks to handle frequencies as low as 1MHz: the ARM generic timer documentation suggests that ARMv8 timers are typically in the 1MHz-50MHz range, which is true on arm64 systems that I have access to. MFC after: 2 weeks Sponsored by: Innovate UK Differential Revision: https://reviews.freebsd.org/D49244 |
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