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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 07:22:51 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:11.0) Gecko/20120329 Thunderbird/11.0.1

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 events.

That means QMP events (like this one) are unreliable. 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 solution is simple--don't use a shared resource for virtio-rng events such that you don't need to worry about rate limiting or event queueing. You process one request, then one piece of data, all over the same socket.


Anthony Liguori

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