From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([208.118.235.92]:41079)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <hpa@zytor.com>) id 1TRqXZ-00021J-Gn
	for qemu-devel@nongnu.org; Fri, 26 Oct 2012 16:30:04 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <hpa@zytor.com>) id 1TRqXY-0001c1-K3
	for qemu-devel@nongnu.org; Fri, 26 Oct 2012 16:30:01 -0400
Received: from terminus.zytor.com ([198.137.202.10]:49418 helo=mail.zytor.com)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <hpa@zytor.com>) id 1TRqXY-0001bv-Bx
	for qemu-devel@nongnu.org; Fri, 26 Oct 2012 16:30:00 -0400
Message-ID: <508AF2C0.30404@zytor.com>
Date: Fri, 26 Oct 2012 13:29:52 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
MIME-Version: 1.0
References: <604401631.2277495.1351264128301.JavaMail.root@redhat.com>
	<871ugl44v5.fsf@codemonkey.ws> <508AB5C0.2000304@zytor.com>
	<508ADD66.5040909@redhat.com>
	<5ea4bbfb-b761-42ef-93f8-7c91fee0bb30@email.android.com>
	<508AE9A6.4060304@redhat.com>
In-Reply-To: <508AE9A6.4060304@redhat.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Subject: Re: [Qemu-devel] [PATCH 0/6] add paravirtualization hwrng support
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Amit Shah <amit.shah@redhat.com>, Anthony Liguori <aliguori@us.ibm.com>, Andreas Faerber <afaerber@suse.de>, qemu-devel@nongnu.org

On 10/26/2012 12:51 PM, Paolo Bonzini wrote:
> Il 26/10/2012 21:07, H. Peter Anvin ha scritto:
>> This is surreal.  Output from /dev/hwrng turns into output for /dev/random... it us guaranteed worse; period, end of story.
> 
> Isn't that exactly what happens in bare-metal?  hwrng -> rngd -> random.  Instead here
> we'd have, host hwrng -> virtio-rng-pci -> guest hwrng -> guest rngd -> guest random.
> 
> The only difference is that you paravirtualize access to the host hwrng to a) distribute
> entropy to multiple guests; b) support migration across hosts with different CPUs and
> hardware.

First, hwrng is only one of the sources used by rngd.  It can also
(currently) use RDRAND or TPM; additional sources are likely to be added
in the future.

Second, the harvesting of environmental noise -- timings -- is not as
good in a VM as on plain hardware, so for the no-hwrng case it is better
for this to be done in the host than in the VM.

	-hpa