So it isn't reading the time from this memory page, it's using TSC. In the case of CLOCK_REALTIME, corrections that are applied to TSC are read from this memory page (comm_page).
This summary only applies to Illumos. The Solaris implementation diverged significantly around build 167 (2011) long after the last OpenSolaris build Illumos was based on (build 147). It changed again significantly in 2015.
I believe Circonus contributed an alternate implementation that does some of the same things as Solaris in 2016:
With that said, you are correct that whether or not it will read from a memory page instead depends on which interfaces you are using (e.g. get_hrusec()) and other subtle details.
This summary only applies to Illumos. The Solaris implementation diverged significantly around build 167 (2011) long after the last OpenSolaris build Illumos was based on (build 147). It changed again significantly in 2015.
I believe Circonus contributed an alternate implementation that does some of the same things as Solaris in 2016:
https://www.circonus.com/2016/09/time-but-faster/
With that said, you are correct that whether or not it will read from a memory page instead depends on which interfaces you are using (e.g. get_hrusec()) and other subtle details.