dtrace/arm64: Fix dtrace_gethrtime()HEADmain

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

0 files changed, 0 insertions, 0 deletions