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Re: [Qemu-devel] [PATCH] rng-random: implement request queue
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH] rng-random: implement request queue |
Date: |
Fri, 5 Feb 2016 09:32:01 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.0 |
>>> 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 <address@hidden>
> 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