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

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Message-ID: <1380620338.645.22.camel@pasglop>
Date:	Tue, 01 Oct 2013 19:38:58 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	Michael Ellerman <michael@...erman.id.au>,
	Paolo Bonzini <pbonzini@...hat.com>,
	linux-kernel@...r.kernel.org, Paul Mackerras <paulus@...ba.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, 2013-10-01 at 11:39 +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.

So for the sake of that dogma you are going to make us do something that
is about 100 times slower ? (and possibly involves more lines of code)

It's not just speed ... H_RANDOM is going to be called by the guest
kernel. A round trip to qemu is going to introduce a kernel jitter
(complete stop of operations of the kernel on that virtual processor) of
a full exit + round trip to qemu + back to the kernel to get to some
source of random number ...  this is going to be in the dozens of ns at
least.

This makes no sense.

Ben.


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