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Re: [Qemu-devel] [PATCH v3 1/1] virtio-rng: hardware random number gener

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH v3 1/1] virtio-rng: hardware random number generator device
Date: Fri, 22 Jun 2012 07:58:53 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:11.0) Gecko/20120329 Thunderbird/11.0.1

On 06/22/2012 07:31 AM, Daniel P. Berrange wrote:
On Fri, Jun 22, 2012 at 07:22:51AM -0500, Anthony Liguori wrote:
On 06/22/2012 07:12 AM, Markus Armbruster wrote:
Anthony Liguori<address@hidden>   writes:

Use a protocol.  This is not what QMP events are designed for!

No human is going to launch nc to a unix domain socket to launch QEMU.
That's a silly use-case to design for.

To be honest, I'm a bit surprised to see working code that got an ACK
>from the guys with the problem it solves rejected out of hand over
something that feels like artistic license to me.

This is an ABI!  We have to support it for the rest of time.
Everything else is a detail that is fixable but ABIs need to not
suck from the beginning.

And using a QMP event here is sucks.  It disappoints me that this is
even something I need to explain.

QMP events occur over a single socket.  If you trigger them from
guest initiated activities (that have no intrinsic rate limit), you
run into a situation where the guest could flood the management tool
and/or queue infinite amounts of memory (because events have to be
queued before they're sent).  So we have rate limiting for QMP

That means QMP events (like this one) are unreliable.

No it doesn't. As it stands currently, the only events that are
rate limited, are those where there is no state information to
loose. ie, the new event completely superceeds the old event
without loosing any information.

                                                       But since QMP
events aren't acked, there's no way for the management tool to know
whether a QMP event was dropped or not.  So you can run into the
following scenario:

- Guest sends randomness request for 10 bytes
- QMP event gets sent for 10 bytes
- Guest sends randomness request for 4 bytes
- QMP is dropped

Now what happens?  With the current virtio-rng, nothing.  It gets
stuck in read() for ever.  Now what do we do?

The RNG event will not be able to use the generic rate limiting
since it has state associated with it. The rate limiting of the
RNG QMP event will need to take account of this state, ie it
will have to accumulate the byte count of any events dropped for
rate limiting:

   - Guest sends randomness request for 10 bytes
   - QMP event gets sent for 10 bytes
   - Guest sends randomness request for 4 bytes
   - QMP is dropped
   - Guest sends randomness request for 8 bytes
   - QMP event gets sent for 12 bytes

BTW, in the current design, there's no way to tell *which* virtio-rng device needs entropy if you have multiple virtio-rng devices.

All of these problems are naturally solved using a protocol over a 


Anthony Liguori


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