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

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Date:	Tue, 1 Oct 2013 18:34:26 +1000
From:	Michael Ellerman <michael@...erman.id.au>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>, gleb@...hat.com,
	agraf@...e.de, mpm@...enic.com, herbert@...dor.apana.org.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 Thu, Sep 26, 2013 at 11:06:59AM +0200, Paolo Bonzini wrote:
> Il 26/09/2013 08:31, Michael Ellerman ha scritto:
> > Some powernv systems include a hwrng. Guests can access it via the
> > H_RANDOM hcall.
> 
> Is there any reason to do this in the kernel?  

It's less code, and it's faster :)

> It does not have to be a particularly fast path;

Sure, but do we have to make it slow on purpose?

> on x86, we are simply forwarding /dev/hwrng or
> /dev/random data to the guest.  You can simply use virtio-rng.

Not all guests support virtio-rng.

> If you really want to have the hypercall, implementing it in QEMU means
> that you can support it on all systems, in fact even when running
> without KVM.  

Sure, I can add a fallback to /dev/hwrng for full emulation.

> The QEMU command line would be something like "-object
> rng-random,filename=/dev/random,id=rng0 -device spapr-rng,rng=rng0".

We can't use /dev/random like that. The PAPR specification, which is
what we're implementing, implies that H_RANDOM provides data from a
hardware source.

cheers
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