> If you want to ensure that all previous instructions are done executing, you need lfence, and if you want to prevent future instructions from filling in the pipelines you'll need an mfence.

LFENCE does not serialize, nor MFENCE. CPUID, however, is documented to as a serializing instruction and is the recommended way to serialize, particularly with RDTSC.

> I don't recall the details, but these different clock states could vary as much as 2.2GHz to 4GHz on my processor (P1? Turbo? I forget the exact name...)

Oh heck, it's way more than that. I've measured ~5x difference in clock cycle count for short loops using RDTSC. Supposedly RDTSC returns "nominal" cycles that advance at the same rate relative to the wall clock, but TBH that doesn't smell right. OSes also try to synchronize the absolute values of the various processors, so jumping between CPUs isn't that bad.