Re: IRQF_SAMPLE_RANDOM question...

Robin Getz
2009-04-09T04:23:27Z

On Wed 8 Apr 2009 19:16, Chris Friesen pondered:
> Gilles Espinasse wrote:
>
> > Readme say :
> > "This daemon attempts to collect real randomness from fluctuations of
> > high-frequency clocks on a PC's mainboard. The basic assumption is that
> > mainboard and CPU are clocked by two separate physical clocks."
>
> > How large is this basic assumption true, on x86, on other arch?
>
> Isn't the cpu frequency normally a phase-locked multiple of the
> mainboard bus frequency?

Yes - typically they are the same.

However - I have tested clrngd out on a Blackfin, and found it gave an
excessively high load - but it did give ok results. 77% of the time (659/848
times) it provided results that passed it's built in FIPS test. It did die a
few times (if the FIPS tests fails 5 times in a row clrngd aborts).

I was going to write my own (based on a similar architecture) - but use the
RTC clock and the main clock - since those actually would be different
physical crystals - and the accuracy of low cost 32kHz crystals is crappy
(typically measureable with a high enough core clock).

But I think delays of cache misses/flushes will dominate things anyway - which
is why clrngd works today on systems which are using the same clock source.
(but since it will be RTC interrupt driven, vs while(1){} like clrngd - the
load will be much lower).

-Robin