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

[Amit Shah <amit.shah@redhat.com>]

On (Thu) 04 Feb 2016 [13:07:35], Ladi Prosek wrote:
> ----- Original Message -----
> > ----- Original Message -----
> > > 
> > > 
> > > On 03/02/2016 13:36, Amit Shah wrote:
> > > > ... 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).
> > > 
> > > 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 :)
> 
> 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()

OK, agreed.

It makes sense to revert this so the live migration scenario is sane.
Without live migration reverting this commit doesn't get us any
benefit, and that's why I had removed it.

Do you want to post a patch reverting this too?

Thanks,

		Amit