access to model specific registers, IA32_APERF / IA32_MPERF, to measure actual CPU frequency

Question

As far as I understand to measure the actual operating CPU frequency I need access to the model specific registers (MSR) IA32_APERF and IA32_MPERF (Assembly CPU frequency measuring algorithm).

However, access to the MSR registers is privileged (through the rdmsr instruction). Is there another way this can be done? I mean, for example, through a device driver/library which I could call in my code. It seems strange to me that reading the registers is privileged. I would think only writing to them would be privileged.

Note: the rdtsc instruction does not account for turbo boost and thus cannot report the actual operating frequency

Edit: I'm interested in solutions for Linux and/or Windows.

I am using Windows 7 and Linux but if I had a solution for either one that would be progress — , Apr 22 '13 at 11:58
For Linux see: http://man7.org/linux/man-pages/man4/msr.4.html — Paul R, Apr 22 '13 at 12:16

score 2 · Accepted Answer · answered Apr 29 '13 at 05:08

2

You are right, the proper way to find average cpu frequency described in 2nd answer in your link.
To read msrs on linux you can use tool RDMSR.
The only thing that maybe missleading in that answer, is maxfrequency. It should be not maxfrequency, but nominal frequency (max non-turbo frequency), as MPERF counter counts in max-non turbo frequency. You can get this frequency from MSR 0xCE bits 8:15 (ref)

answered Apr 29 '13 at 05:08

Yevgeni

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Thank you. I was wondering about what to do with maxfrequency since it's part of what I am trying to determine. Since it's really the nominal frequency I think I know what to do now. – Apr 29 '13 at 08:01

access to model specific registers, IA32_APERF / IA32_MPERF, to measure actual CPU frequency

1 Answers1