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

[Paolo Bonzini]

Il 26/10/2012 22:29, 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.

Neither of these make /dev/random with virtio-rng-pci worse than without
(as would be the case if you fed /dev/urandom).  And migration works.
This, and avoiding denial of service for the host's /dev/random, is all
I care about at this time.

There is always time to change defaults to something better.

Paolo