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.s...@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