Messages in this thread | | | Date | Tue, 04 Feb 2014 13:34:40 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [RFC PATCH 0/5] CPU Jitter RNG |
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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.
-hpa
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