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Nice article!

If you're interested in clocks on Linux, you might also find this article useful (shameless plug): http://btorpey.github.io/blog/2014/02/18/clock-sources-in-li...

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> Note that the 100ns mentioned above is largely due to the fact that my Linux box doesn’t support the RDTSCP instruction, so to get reasonably accurate timings it’s also necessary to issue a CPUID instruction prior to RDTSC to serialize its execution.

Huh? That’s definitely not true now, and I don’t think it ever was. Linux uses LFENCE or MFENCE, depending on CPU.

Using CPUID as a serializing instruction before RDTSC{,P} is a bad bad thing to do inside a virtual machine on Intel processors. CPUID will cause a VMEXIT, and the CPUID instruction will be emulated. The Intel Software Development Manual Instruction Set Reference gives good guidance on using MFENCE and LFENCE as required.

https://software.intel.com/sites/default/files/managed/39/c5...

Linux has mostly stopped using CPUID to serialize at all. When full serialization is needed, we use IRET now. In the future, we could optimize a bit by writing to CR2, except on Xen.

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