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

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Message-ID: <20140204163957.GA13928@order.stressinduktion.org>
Date:	Tue, 4 Feb 2014 17:39:57 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Stephan Mueller <smueller@...onox.de>
Cc:	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 05:19:52PM +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?

Greetings,

  Hannes

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