linux-kernel - Re: [PATCH 2/5] CPU Jitter RNG: Enable compilation

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Date:	Tue, 4 Feb 2014 17:50:33 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Stephan Mueller <smueller@...onox.de>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Theodore Ts'o <tytso@....edu>,
	Jörn Engel <joern@...fs.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	"Maciej W. Rozycki" <macro@...ux-mips.org>,
	Ralf Baechle <ralf@...ux-mips.org>, dave.taht@...il.com,
	John Crispin <blogic@...nwrt.org>, andrewmcgr@...il.com,
	Thorsten Glaser <tg@...bsd.de>, sandyinchina@...il.com
Subject: Re: [PATCH 2/5] CPU Jitter RNG: Enable compilation

On Tue, Feb 04, 2014 at 05:39:57PM +0100, Hannes Frederic Sowa wrote:
> On Tue, Feb 04, 2014 at 05:19:52PM +0100, Stephan Mueller wrote:
> > Also, I consider the execution speed of the entropy collection is not 
> > really an issue because the RNG delivers random numbers at a 
> > comparatively high rate. Any other noise source feeding into random.c 
> > delivers data with far less speed.
> 
> Compiling the kernel with -O0 could add some other problems, like
> e.g. not doing enough constant folding which could result in linking
> errors. I guess it is not a problem currently though, but some of the
> compile time checks depend on this (compiletime_assert and such).
> 
> Have you looked into adding compiler barriers into relevant places in the
> loops to stop the compiler from optimizing and spill out the values from the
> registers to their memory locations?

Quick follow-up:

Maybe you can get some ideas how to stop the compiler from optimizing code
from commit fe8c8a126806fe ("crypto: more robust crypto_memneq"). Maybe also
volatile could be helpful and OPTIMIZER_HIDE_VAR seems to be a good candidate
to use here, too.

Greetings,

  Hannes

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