linux-kernel - Re: [PATCH 1/2] random: Add support for architectural random hooks

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Date:	Sat, 30 Jul 2011 15:13:27 -1000
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Matt Mackall <mpm@...enic.com>,
	Herbert Xu <herbert@...dor.hengli.com.au>,
	"Theodore Ts'o" <tytso@....edu>, Jeff Garzik <jgarzik@...ox.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] random: Add support for architectural random hooks

So here is my counter-suggestion

NOTE NOTE NOTE! This is completely and utterly untested.  I didn't
actually check how big the "rdrand" and "setc" instructions are, so
the ASM_NOP4 there is just a random "I guess two 'xor' instructions
are four bytes shorter than the rdrand/setc instructions are".

So please don't take this as a serious patch that should be applied,
but instead take it as a serious alternative *approach*.

Note that with the default inline function in <asm-generic/random.h>
is designed so that architectures that use it (this patch does *not*
contain the architecture glue to enable it) will compile the loop in
random.c entirely away. No test, no nothing.

Comments?

(Btw, even on x86, assuming the concept works and the ASM_NOP4 is
corrected to the right length, we'd need to support older assemblers
that don't understand the "rdrand" instruction, so it would need to be
done as a explicit byte sequence).

Again, TOTALLY UNTESTED. Concept patch only!! There may be seriously
stupid bugs here, but the point is that it should make it easy for an
architecture to have a "get a word of random data quickly".

                     Linus

View attachment "patch.diff" of type "text/x-patch" (2402 bytes)