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Subject: Re: [PATCH 3/3] KVM: PPC: Book3S: Add support for hwrng found on
 some powernv systems
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Gleb Natapov <gleb@redhat.com>
Date: Thu, 03 Oct 2013 08:21:20 +1000
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Cc: tytso@mit.edu, kvm@vger.kernel.org, linuxppc-dev@ozlabs.org,
 Alexander Graf <agraf@suse.de>, kvm-ppc@vger.kernel.org,
 linux-kernel@vger.kernel.org, herbert@gondor.hengli.com.au,
 Paul Mackerras <paulus@samba.org>, mpm@selenic.com,
 Paolo Bonzini <pbonzini@redhat.com>
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On Wed, 2013-10-02 at 17:37 +0300, Gleb Natapov wrote:
> On Wed, Oct 02, 2013 at 04:33:18PM +0200, Paolo Bonzini wrote:
> > Il 02/10/2013 16:08, Alexander Graf ha scritto:
> > > > The hwrng is accessible by host userspace via /dev/mem.
> > > 
> > > A guest should live on the same permission level as a user space
> > > application. If you run QEMU as UID 1000 without access to /dev/mem, why
> > > should the guest suddenly be able to directly access a memory location
> > > (MMIO) it couldn't access directly through a normal user space interface.
> > > 
> > > It's basically a layering violation.
> > 
> > With Michael's earlier patch in this series, the hwrng is accessible by
> > host userspace via /dev/hwrng, no?
> > 
> Access to which can be controlled by its permission. Permission of
> /dev/kvm may be different. If we route hypercall via userspace and
> configure qemu to get entropy from /dev/hwrng everything will fall
> nicely together (except performance).

Yes, except abysmall performance and a lot more code for something
completely and utterly pointless .... nice.

Ben.