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

From: Benjamin Herrenschmidt <[email-masked]>
Date: Tue, 01 Oct 2013 19:38:58 +1000

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.