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I want to check if the RDTSC instruction is available. There must be a Intel Pentium or newer processor and either the TSD flag in register CR4 is clear or it is set and the CPL equals 0.

So, there's no problem to obtain the current privilege level (Bits 0 and 1 of the CS segment register). Also there is no problem to check if the instruction itself is supported (CPUID.1:EDX[4] = 1).

But (and that's the problem), this must also run under user-mode (PL3). But, I can't read the control register CR4 in user-mode.

Is there any other way to check if the operation system does restrict the access to the time-stamp counter?

Peter Cordes
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0xbadf00d
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1 Answers1

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The only way is to "try" the instruction and intercept the exception, provided that the operating system gives you the ability to react to the event in a safe way and recover your state so you can continue your program. Unfortunately not every OS permits to continue after an exception that it considers "fatal". On Windows you can try to play with the structured exception handling, on linux there are specific signals (SIGILL, in particular). But other OSes don't forgive this kind of exceptions.
Bye

(edit)

PS: it's also possible, in principle, for an OS to intercept the exception and simulate the instruction so the application has no way to decide if the instruction is really available. I don't know if there are OSes that do this thing (virtual machines, maybe?).
Bye!

Sep Roland
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Giuseppe Guerrini
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