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 12:57:59 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Michael Ellerman <michael@...erman.id.au>,
	Paolo Bonzini <pbonzini@...hat.com>,
	linux-kernel@...r.kernel.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	agraf@...e.de, 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 Tue, Oct 01, 2013 at 07:23:20PM +1000, Paul Mackerras wrote:
> On Tue, Oct 01, 2013 at 11:39:08AM +0300, Gleb Natapov wrote:
> > On Tue, Oct 01, 2013 at 06:34:26PM +1000, Michael Ellerman wrote:
> > > 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?
> > > 
> > We do not put non performance critical devices into the kernel.
> 
> It's not a device, it's a single hypercall, specified by PAPR, which
> is the moral equivalent of x86's RDRAND.
> 
OK. A couple of general questions. How guest knows when this hypercall
is available? Is QEMU enable it when KVM_CAP_PPC_HWRNG is available (I
haven't seen userspace patch that uses KVM_CAP_PPC_HWRNG)? What about
QEMU TCG does it implement this hypercall or it emulates hwrng directly?

--
			Gleb.
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