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:

quoted

On Thu, Sep 26, 2013 at 11:06:59AM +0200, Paolo Bonzini wrote:

quoted

Il 26/09/2013 08:31, Michael Ellerman ha scritto:

quoted

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 :)

quoted

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.