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