Re: [Qemu-devel] [PATCH 0/6] add paravirtualization hwrng support

[H. Peter Anvin]

On 10/26/2012 12:51 PM, Paolo Bonzini wrote:
> Il 26/10/2012 21:07, H. Peter Anvin ha scritto:
>> This is surreal.  Output from /dev/hwrng turns into output for 
>> /dev/random... it us guaranteed worse; period, end of story.
> 
> Isn't that exactly what happens in bare-metal?  hwrng -> rngd -> random.  
> Instead here
> we'd have, host hwrng -> virtio-rng-pci -> guest hwrng -> guest rngd -> guest 
> random.
> 
> The only difference is that you paravirtualize access to the host hwrng to a) 
> distribute
> entropy to multiple guests; b) support migration across hosts with different 
> CPUs and
> hardware.

First, hwrng is only one of the sources used by rngd.  It can also
(currently) use RDRAND or TPM; additional sources are likely to be added
in the future.

Second, the harvesting of environmental noise -- timings -- is not as
good in a VM as on plain hardware, so for the no-hwrng case it is better
for this to be done in the host than in the VM.

        -hpa