linux-kernel - Re: [Resend PATCH 2/2] s390: provide hardware randomness from zcrypt card to /dev/random

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Date:	Thu, 12 Sep 2013 13:37:53 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Torsten Duwe <duwe@....de>
CC:	tytso@....edu, ingo.tuchscherer@...ibm.com,
	linux-kernel@...r.kernel.org,
	Hans-Georg Markgraf <MGRF@...ibm.com>,
	Gerald Schaefer <gerald.schaefer@...ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Joe Perches <joe@...ches.com>
Subject: Re: [Resend PATCH 2/2] s390: provide hardware randomness from zcrypt
 card to /dev/random

On 09/12/2013 02:41 AM, Torsten Duwe wrote:
> 
> Running completely virtualised, system Z severely lacks good true random sources.
> Gathering entropy in a virtual environment is difficult. To compensate, there is
> specialised crypto hardware which includes a source for hardware randomness;
> the zcrypt driver is able to access this random source. This patch adds a kernel
> thread that feeds the random bits via the interface created with the previous patch.
> 
> Signed-off-by: Torsten Duwe <duwe@....de>

>From what I can gather from the patch this is too heavyweight (need
locks and so on) to use as arch_get_random*().  There has been a lot of
discussion about the pros and cons of allowing the kernel to bypass
rngd, but I would think that any such plumbing -- once it gets past the
fully synchronous low latency properties of arch_get_random*() -- really
should be implemented as an option in the existing hwrng device
infrastructure.

In other words, start by implementing a hwrng device.  That will work
right now with rngd running.  Then we can consider if we want to allow
bypass of rngd for certain hwrng devices -- which may include zcrypt,
virtio_rng and so on.

	-hpa


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