linux-kernel - Re: [PATCH 2/2] lguest: virtio-rng support

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Date:	Mon, 19 May 2008 10:45:23 +0100
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	"H. Peter Anvin" <hpa@...or.com>, Theodore Tso <tytso@....edu>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Jeff Garzik <jeff@...zik.org>,
	LKML <linux-kernel@...r.kernel.org>,
	virtualization@...ts.linux-foundation.org,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Matt Mackall <mpm@...enic.com>,
	Johannes Berg <johannes@...solutions.net>
Subject: Re: [PATCH 2/2] lguest: virtio-rng support

Rusty Russell wrote:
> Unfortunately not.  Hardware randomness devices export /dev/hwrng, and it's up 
> to userspace to feed that into /dev/random (or not).  That's usually done by 
> rngd, which at least on my system, assumes 1 bit of entropy per bit of data 
> from /dev/hwrng by default.
>
> I was a little surprised that this decision was exported to userspace, but if 
> you're not prepared to unconditionally trust hw rngs, it makes sense to palm 
> it off.

Yeah, that's a bit of a pity.  Hardware rngs can often generate really 
crappy randomness, which needs tons of processing to remove noise like 
50/60hz hum, etc.

>   We could write a boutique device for virtualization which *did* feed 
> directly, but that would be a little gauche.
>   

Well, yes, we can certainly do any amount of processing we like to the 
stuff provided to guests, so that the 1:1 bits/entropy ratio is as true 
as we can make it.

    J
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