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Date: Fri, 27 Sep 2013 10:58:40 +0200
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Subject: Re: [Qemu-devel] [PATCH] spapr: Add support for hwrng when available
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To: Michael Ellerman <michael@ellerman.id.au>
Cc: Paul Mackerras <paulus@samba.org>, Alexander Graf <agraf@suse.de>, qemu-devel@nongnu.org

Il 27/09/2013 10:36, Michael Ellerman ha scritto:
> So have the host kernel read from the hwrng, export it to userspace via
> a char device, which is then read by qemu, which then copies it back
> into the host kernel, which can then give it to the guest.
> 
> Or from the guest perspective, instead of a cheap switch to host real
> mode and back - a full switch to kernel virtual, then to user, back to
> kernel, back to user, back to kernel, back to guest.
> 
> Frankly I can't see why that is a superior option?

Because this is not a fast path at all.  Doing things in QEMU lets
people test and configure the paravirtualized hwrng even if they do not
have a hwrng in the host, and even if they are running with emulation
(TCG) instead of KVM.

But as I mentioned in the kernel thread, perhaps you do not need the
hypercall at all if virtio-rng is enough (it should be for Linux guests).

Paolo