Re: [Qemu-devel] [PATCH] rng-random: implement request queue

[Paolo Bonzini <pbonzini@redhat.com>]

>>> It is already migrated as part of virtio_rng_save's call to virtio_save.
>>>  On the loading side, virtio_rng_process condenses all requests into one
>>> and chr_read fills in as many virtqueue buffers as possible from the
>>> single request.
>>
>> Thanks! So this looks broken. The contract between virtio-rng and the rng
>> backend is "one chr_read callback per one rng_backend_request_entropy",
>> regardless of the request size. It guarantees that chr_read will get at
>> least one byte, nothing else. If the one condensed request is bigger than
>> what the backend is able to give at the moment, there may be unsatisfied
>> virtio buffers left in the queue.
>>
>> One way of fixing this would be to have chr_read call virtio_rng_process
>> if it ran out of backend data but not out of virtio buffers. Otherwise
>> those buffers will be slowly drained by the rate limit timer, which is
>> not optimal (especially in the absence of rate limit :)

Nice observation!  And the commit you mention below seems to be the
right way to fix it indeed.  Amit, do you recall why this was done?

> Basically, we could just revert this commit:
> 
> commit 4621c1768ef5d12171cca2aa1473595ecb9f1c9e
> Author: Amit Shah <amit.shah@redhat.com>
> Date:   Wed Nov 21 11:21:19 2012 +0530
> 
>     virtio-rng: remove extra request for entropy
>     
>     If we got fewer bytes from the backend than requested, don't poke the
>     backend for more bytes; the guest will ask for more (or if the guest has
>     already asked for more, the backend knows about it via handle_input()

I've now reviewed your patch more carefully, I will make some comments
in reply to the patch directly.

Paolo