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

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Date:	Tue, 04 Feb 2014 13:34:40 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Stephan Mueller <smueller@...onox.de>
CC:	"Theodore Ts'o" <tytso@....edu>,
	Jörn Engel <joern@...fs.org>,
	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	macro@...ux-mips.org, ralf@...ux-mips.org, dave.taht@...il.com,
	blogic@...nwrt.org, andrewmcgr@...il.com, geert@...ux-m68k.org,
	tg@...bsd.de, sandyinchina@...il.com
Subject: Re: [RFC PATCH 0/5] CPU Jitter RNG

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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