linux-kernel - Re: enable dual rng on VIA C7

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Message-ID: <20071127185053.GA30057@redhat.com>
Date:	Tue, 27 Nov 2007 13:50:53 -0500
From:	Dave Jones <davej@...hat.com>
To:	Udo van den Heuvel <udovdh@...all.nl>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Michael Buesch <mb@...sch.de>,
	folkert van Heusden <folkert@...heusden.com>
Subject: Re: enable dual rng on VIA C7

On Tue, Nov 27, 2007 at 05:08:26PM +0100, Udo van den Heuvel wrote:
 > Dave Jones wrote:
 > > On Mon, Nov 26, 2007 at 06:02:39PM +0100, Udo van den Heuvel wrote:
 > > 
 > >  > I did not know we are already that far ;-)
 > >  > I mean: can this patch be aplied without hurting C3/C7 CPU's with just
 > >  > one RNG? Maybe an expert needs to test/answer?
 > >  > Maybe some logic needs to be applied around the extra bit?
 > >  
 > >>From the padlock spec..
 > > 
 > > "SRC Bits[9:8] Noise source select (I): These bits control the two noise
 > >  sources on the processor that input bits to the accumulation buffers.
 > >  On Nehemiah processors prior to stepping 8, these bits are reserved
 > >  and undefined. The default RESET state is both bits = 0." 
 > > 
 > > Something like this perhaps ?
 > 
 > Yes, I think that's a big step in the right direction!
 > 
 > But I am no expert and cannot really judge how necessary or correct the
 > implementation is w.r.t. the 'undefined' function bits for CPU's that
 > lack a certain feature.

The checks at the end of the patch for the x86_mask/model ensure
we only enable the 2nd noise source on CPUs documented to have it,
so we should be safe.

Andrew, want to throw that in the -mm pile for a while?

	Dave

-- 
http://www.codemonkey.org.uk
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