On Wed, 2008-10-01 at 17:39 -0700, H. Peter Anvin wrote: > third, which is subject to spread-spectrum modulation due to RFI > concerns. Therefore, relying on the *nominal* frequency of this clock I'm not suggesting using the nominal value. I'm suggesting the measurement be done in the one and only place where there is perfect control of the system, the processor boot-strapping in the BIOS. Only the platform designers themselves know the speed of the oscillator which is modulating the clock and so only they should be calibrating the speed of the TSC. If this modulation really does alter the frequency by +/- 2% (seems high to me, but hey, I don't design motherboards), using an LFO, then basically all the calibration done in Linux is broken and has been for some time. You can't calibrate only once, or risk being off by 2%, you can't calibrate repeatedly and take the fastest estimate, or you are off by 2%, and you can't calibrate repeatedly and take the average without risking SMI noise affecting the lowest clock speed measurement, contributing unknown error. Hmm. Re-reading your e-mail, I see you are saying the nominal frequency may be off by 2% (and I easily believe that), not necessarily that the frequency modulation may be 2% (which I still think is high). Does anyone know what the actual bounds on spread spectrum modulation are or how fast the clock is modulated? Zach _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/virtualization
