From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756854AbYESJ23 (ORCPT ); Mon, 19 May 2008 05:28:29 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751692AbYESJ2T (ORCPT ); Mon, 19 May 2008 05:28:19 -0400 Received: from ozlabs.org ([203.10.76.45]:34310 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751173AbYESJ2T (ORCPT ); Mon, 19 May 2008 05:28:19 -0400 From: Rusty Russell To: Jeremy Fitzhardinge Subject: Re: [PATCH 2/2] lguest: virtio-rng support Date: Mon, 19 May 2008 19:28:12 +1000 User-Agent: KMail/1.9.9 Cc: "H. Peter Anvin" , Theodore Tso , Herbert Xu , Jeff Garzik , LKML , virtualization@lists.linux-foundation.org, Christian Borntraeger , Matt Mackall , Johannes Berg References: <482C8595.5030509@garzik.org> <200805191905.11452.rusty@rustcorp.com.au> <48314419.4080606@goop.org> In-Reply-To: <48314419.4080606@goop.org> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200805191928.13043.rusty@rustcorp.com.au> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Monday 19 May 2008 19:10:49 Jeremy Fitzhardinge wrote: > Rusty Russell wrote: > > On Saturday 17 May 2008 17:47:01 Jeremy Fitzhardinge wrote: > >> But the host->guest protocol should include the number of bits estimated > >> entropy along with the bits themselves. > > > > If we go down that path, we have to come up with a way of feeding that > > information to guest userspace. Patches welcome. If we meanwhile assume > > entropy == #bits, it fits the current /dev/hwrng model, and we can add an > > entropy count later if we want to change that. > > Really? Wouldn't guest userspace just use /dev/random as usual, which I > think does have an ioctl to give that kind of information? And the > interfaces do allow things injecting entropy into the kernel pool to > provide entropy estimates along with the actual bits themselves. 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. We could write a boutique device for virtualization which *did* feed directly, but that would be a little gauche. Cheers, Rusty.