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

[Ladi Prosek]

----- 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 <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()

> > Cancel_requests seems useless.
> 
> I'll delete that code.
> 
> > Paolo
> > 
> > 
> 
>