Re: [Qemu-devel] [PATCH 2/2] VirtIO RNG

[Paul Brook]

>> >        This patch adds support for virtio-rng. Data is read from a
>> > chardev and can be either raw entropy or received via the EGD protocol.
>>
>> I still don't get why you need this at all. It seems like
>> virtio-serial would already provides everything you need.
>
>I guess when virtio-rng was first written, virtio-serial wasn't
>flexible enough because it didn't support multiple devices 

That argument no longer holds.

> and maybe virtio-rng is still needed to enforce the EGD protocol when that
> is being used.

Maybe, though the benefit of having this knowledge in wemu seems somewhat 
unclear. If we do want it then shouldn't be be implemented as a char device 
backend, rather than part of a specific serial port implementation?

> > > +        qemu_gettimeofday(&now);
> >
> > Using qemu_gettimeofday is almost certainly wrong, and you want to
> > be using virtual time.  Plus I'm not convinced this is the right
> > place to enforce rate limiting.
> 
> If it's for rate limiting how fast the guest can take entropy from the
> host, to ensure the host's entropy cannot be exhausted by a single
> greedy guest, then perhaps qemu_gettimeofday() is right here.  I doubt
> if virtual time is right, at least not by itself.

gettimeofday can and does jump arbitrarily. Comparing returned values is 
almost always wrong.

Paul