So the only things I'm seeing in the linked circonus code that differ from illumos:

1. no use of a kernel supplied page, determines skew/etc itself in userspace 2. stores information on a per-cpu level, and tries to execute cpuid on the same cpu as rdtsc.

I'm presuming you're talking about #2 (and #1 is just due to the linked item being a library without kernel integrations)? Perhaps with some more kernel support so that the actual cpu rdtsc ran on can be reliably determined?

This still doesn't clarify the part about "shared page in which the time is updated" and is read from. This statement appears to imply TSC is not (necessarily) used (otherwise I'd categorize it under "uses values from memory page to fixup TSC", like Illumos' current implimentation). I'm still not sure how that can be done reasonably.

Is there just a 1 micro second timer running whenever a user task is being executed that is bumping the value? Wouldn't that be quite a bit of overhead? Or some HW trick? I mean, you could generate a fault on every read, and have the kernel populate the current data, but that seems just as bad as a syscall.