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Date: Thu, 1 Apr 2010 13:30:36 +0100
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] [PATCH 2/2] VirtIO RNG
Message-ID: <20100401123036.GA1752@shareable.org>
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	<201004011317.45269.paul@codesourcery.com>
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To: Paul Brook <paul@codesourcery.com>
Cc: Ian Molton <ian.molton@collabora.co.uk>, qemu-devel@nongnu.org, Gerd Hoffmann <kraxel@redhat.com>

Paul Brook wrote:
> >        This patch adds support for virtio-rng. Data is read from a chardev
> > and can be either raw entropy or received via the EGD protocol.
> 
> I still don't get why you need this at all. It seems like
> virtio-serial would already provides everything you need.

I guess when virtio-rng was first written, virtio-serial wasn't
flexible enough because it didn't support multiple devices - and maybe
virtio-rng is still needed to enforce the EGD protocol when that is
being used.

> > +        qemu_gettimeofday(&now);
> 
> Using qemu_gettimeofday is almost certainly wrong, and you want to
> be using virtual time.  Plus I'm not convinced this is the right
> place to enforce rate limiting.

If it's for rate limiting how fast the guest can take entropy from the
host, to ensure the host's entropy cannot be exhausted by a single
greedy guest, then perhaps qemu_gettimeofday() is right here.  I doubt
if virtual time is right, at least not by itself.

I would hope that the host can rate limit itself without needing apps
to govern themselves, though.

-- Jamie