Why isn't RDTSC a serializing instruction?

Question

The Intel manuals for the RDTSC instruction warn that out of order execution can change when RDTSC is actually executed, so they recommend inserting a CPUID instruction in f

Answer 1

why didn't they make RDTSC serializing? The entire point of it appears to be to get cycle accurate timings

Well, most of the time it's to get high-resolution timestamps. At least some of the time, these timestamps are used for performance metrics. Making the intruction serializing would likely require a pipeline flush, which can be very expensive for CPU-bound applications.

Intel opted to introduce a separate instruction rather than change the behavior of RDTSC, which suggests to me that there has to be some situation where a potentially out of order timing is what you want.

Changing the behavior is almost always undesirable. Intel's customers would be disappointed to find out that RDTSC does something different on newer parts.