linux-kernel - Re: [PATCH 3/3] KVM: PPC: Book3S: Add support for hwrng found on some powernv systems

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Date:	Thu, 03 Oct 2013 08:02:20 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	Alexander Graf <agraf@...e.de>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <michael@...erman.id.au>,
	linux-kernel@...r.kernel.org, mpm@...enic.com,
	herbert@...dor.hengli.com.au, linuxppc-dev@...abs.org,
	kvm@...r.kernel.org, kvm-ppc@...r.kernel.org, tytso@....edu
Subject: Re: [PATCH 3/3] KVM: PPC: Book3S: Add support for hwrng found on
 some powernv systems

On Wed, 2013-10-02 at 13:02 +0300, Gleb Natapov wrote:

> Yes, I alluded to it in my email to Paul and Paolo asked also. How this
> interface is disabled? Also hwrnd is MMIO in a host why guest needs to
> use hypercall instead of emulating the device (in kernel or somewhere
> else?).

Migration will have to be dealt with one way or another, I suppose we
will indeed need a qemu fallback.

As for why hypercall instead of MMIO, well, you'd have to ask the folks
who wrote the PAPR spec :-) It's specified as a hypercall and
implemented as such in pHyp (PowerVM). The existing guests expect it
that way.

It might have to do with the required whitening done by the hypervisor
(H_RANDOM output is supposed to be clean). It also abstracts us from the
underlying HW implementation which could in theory change.
 
>  Another things is that on a host hwrnd is protected from
> direct userspace access by virtue of been a device, but guest code (event
> kernel mode) is userspace as far as hosts security model goes, so by
> implementing this hypercall in a way that directly access hwrnd you
> expose hwrnd to a userspace unconditionally. Why is this a good idea? 

Why would this be a bad idea ?

Ben.

> --
> 			Gleb.
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