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

[Amit Shah]

Hi Ladi,

Adding Pankaj to CC, he too looked at this recently.

On (Fri) 22 Jan 2016 [13:19:58], Ladi Prosek wrote:
> If the guest adds a buffer to the virtio queue while another buffer
> is still pending and hasn't been filled and returned by the rng
> device, rng-random internally discards the pending request, which
> leads to the second buffer getting stuck in the queue. For the guest
> this manifests as delayed completion of reads from virtio-rng, i.e.
> a read is completed only after another read is issued.
> 
> This patch adds an internal queue of requests, analogous to what
> rng-egd uses, to make sure that requests and responses are balanced
> and correctly ordered.

... and this can lead to breaking migration (the queue of requests on
the host needs to be migrated, else the new host will have no idea of
the queue).

I think we should limit the queue size to 1 instead.  Multiple rng
requests should not be common, because if we did have entropy, we'd
just service the guest request and be done with it.  If we haven't
replied to the guest, it just means that the host itself is waiting
for more entropy, or is waiting for the timeout before the guest's
ratelimit is lifted.

So, instead of fixing this using a queue, how about limiting the size
of the vq to have just one element at a time?

Thanks,

                Amit