Re: [Qemu-devel] [PATCH 0/8] add paravirtualization hwrng support (v2)

[Aurelien Jarno <aurelien@aurel32.net>]

On Tue, Oct 30, 2012 at 06:24:27PM -0700, H. Peter Anvin wrote:
> On 10/30/2012 04:02 PM, Anthony Liguori wrote:
> >
> >My take away from all of the various discussions on what the Right Way to
> >use virtio-rng is:
> >
> >  1) /dev/random should always be used as the entropy source (I've left it
> >     configurable though)
> >
> >  2) I think the Right Way to configure virtio-rng is to figure out what the
> >     available entropy is on the host, and then decide how to allocate that
> >     to each guest.  As such, I've implemented rate limiting.
> >
> >     I think QEMU is the right place to do this because this is a property of
> >     specific virtual machines.  I can imagine a cloud provider wanting to
> >     guarantee a certain level of entropy for different classes of VMs.  Even
> >     if rngd could be used to do this, configuring it differently for different
> >     guests would be cumbersome.
> >
> 
> rngd is not where this should happen, it should be in the
> /dev/random implementation in the (host) kernel.  That way it is
> applicable to all types of clients, not just Qemu.
> 
> >  3) `qemu -device virtio-rng-pci` will Just Work but risks exhausting host
> >     entropy.  This means we can't make it the default for machines.  But for
> >     most command line users, I think this is the behavior they want.
> 
> It's a bit unfortunate, but I'm not going to push on that point.
> 
> Given the migration issue I'll write up an implementation of a DRNG
> (RDRAND/RDSEED) backend once this is upstream.  If RDRAND is
> disabled in the guest, but available in the host, this would be the
> one to use.  If RDRAND is available in the guest it should be used
> directly if rngd is new enough, but since virtio-rng has been in the
> kernel since 2008 there still might be some guests which could use
> such an implementation without having been RDRAND-enabled.
> 

That is also a good idea for emulation, and especially to provide
non-x86 guests with entropy.

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net