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Asked: 2026-05-26T05:37:42+00:00

I am studying RDTSC and learning about how it is virtualized for the purposes

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I am studying RDTSC and learning about how it is virtualized for the purposes of virtual machines like VirtualBox and VMWare. Why did Intel/AMD go to all the trouble of virtualizing this instruction?

I feel like it can be easily simulated with a trap and it’s not exactly a super-common instruction (I tested and there’s no noticable slow-down for general usage in a virtual machine where hardware RDTSC virtualization is disabled).

However, I know Intel/AMD wouldn’t have gone to all the trouble to add this instruction to the virtualizing hardware unless it was important to able to execute very fast.

Does anyone know why?

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1 Answer

  1. Editorial Team

    Its common to use RDTSC to get fine-grained timing information, where the overhead of a virtualization trap would be quite significant. Most common use is to have two RDTSC instructions with a small amount of code between them, taking the difference of the times as the elapsed time (number of cycles) for the code sequence. So even the overhead of pipeline drains/flushes is quite significant.

    Also, since all the instruction does is read a continuously running counter, virtualizing it is quite easy — the hardware only needs to allow saving/reloading the counter value on VM context switches, and not anything special for the RDTSC instruction itself.

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