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Re: [Qemu-discuss] [libvirt-users] Efficacy of jitterentropy RNG on qemu

From: Daniel P . Berrangé
Subject: Re: [Qemu-discuss] [libvirt-users] Efficacy of jitterentropy RNG on qemu-kvm Guests
Date: Thu, 16 Aug 2018 14:24:35 +0100
User-agent: Mutt/1.10.1 (2018-07-13)

On Fri, Aug 10, 2018 at 08:33:00PM +0000, procmem wrote:
> Hello. I'm a distro maintainer and was wondering about the efficacy of
> entropy daemons like haveged and jitterentropyd in qemu-kvm. One of the
> authors of haveged [0] pointed out if the hardware cycles counter is
> emulated and deterministic, and thus predictible. He therefore does not
> recommend using HAVEGE on those systems. Is this the case with KVM's
> counters?
> PS. I will be setting VM CPU settings to host-passthrough.

Hardware from circa 2011 onwards has RDRAND support, and with host-passthrough
this will be available to the guest.  The rngd daemon, running in the guest,
can use this as a source to feed the kernel entropy.

In addition QEMU has support for virtio-rng which can pull entropy from
/dev/urandom on the host, and feed it into the guest, where again rngd can
give it to the kernel.

So why do you need to consider haveged / jitterentropyd at all with QEMU ?
It should suffice to just enable virtio-rng in the host and run rngd in
all guests. If the host has RDRAND, that's an extra bonus.

haveged / jitterentropyd should only be needed on other non-QEMU hypervisors
which don't support something equiv to virtio-rng, and are on hardware that
is too old for RDRAND.

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