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Date:	Thu, 07 Aug 2014 12:31:11 -0700
From:	"H. Peter Anvin" <>
To:	Amit Shah <>,
CC:	Virtualization List <>,
	Amos Kong <>,
	Rusty Russell <>,,
	Ricardo Neri <>
Subject: Re: [PATCH 1/2] rngd: add udev rule to source from hwrng if virtio-rng

On 08/07/2014 06:08 AM, Amit Shah wrote:
> On KVM guests where the virtio-rng device is available, and set as the
> current rng, this udev rule will start rngd which will feed in the
> host-provided entropy to /dev/random.
> Signed-off-by: Amit Shah <>
> ---
>  90-virtio-rng.rules | 1 +
>  1 file changed, 1 insertion(+)
>  create mode 100644 90-virtio-rng.rules
> diff --git a/90-virtio-rng.rules b/90-virtio-rng.rules
> new file mode 100644
> index 0000000..07f2f9c
> --- /dev/null
> +++ b/90-virtio-rng.rules
> @@ -0,0 +1 @@
> +KERNEL=="hw_random", ATTR{rng_current}=="virtio*", RUN+="/sbin/rngd -r /dev/hwrng"

I'm not sure this is really right.  rngd can receive input from a number
of sources, and I suspect it would be better to rngd to be started much
earlier and perhaps get sent a SIGHUP to reconfigure when a hw_random
device appears ... not just the virtio one.

Currently rngd exits if there is no daemon.  There are a few ways to
deal with that: one is to run rngd on changes and leave a pid file
around; the rngd can try to signal the old rngd or launch itself;
another option would be to have a different rngd for each source.  To
some degree the above is obsolete when we get khwrngd widely deployed,
but that is a new-kernel-only kind of thing.


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