Re: [RFC PATCH 0/5] CPU Jitter RNG

From: H. Peter Anvin
Date: Tue Feb 04 2014 - 16:35:29 EST

On 02/04/2014 12:31 PM, Stephan Mueller wrote:
>> The quantum noise sources there are in a system are generally two
>> independent clocks running against each other. However, independent
>> clocks are rare; instead, most clocks are in fact slaved against each
>> other using PLLs and similar structures. When mixing spread spectrum
>> clocks and non-spread-spectrum clocks that relationship can be very
>> complex, but at least for some designs it is still at its core
>> predictable.
> But isn't there an additional clock? The clock used to drive the cache
> and memory bus? When measuring memory accesses timings, larger
> variations in the execution time are evident. This also applies when
> hitting the caches (for L1, the variations are less than for L2 than for
> L3). The variations in access timings would come from the CPU wait
> states and their duration, would it not?

Variations doesn't mean quantum unpredictable noise. All the clocks you
are referring to are derived from the same BCLK and thus predictable.
What you have here is a PRNG with a large and obscure state space.


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