From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:41996) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aROJg-0004cQ-Ut for qemu-devel@nongnu.org; Thu, 04 Feb 2016 13:07:41 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1aROJd-0001Cl-OB for qemu-devel@nongnu.org; Thu, 04 Feb 2016 13:07:40 -0500 Received: from mx5-phx2.redhat.com ([209.132.183.37]:35075) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aROJd-0001Cd-GZ for qemu-devel@nongnu.org; Thu, 04 Feb 2016 13:07:37 -0500 Date: Thu, 4 Feb 2016 13:07:35 -0500 (EST) From: Ladi Prosek Message-ID: <65843372.32316615.1454609255745.JavaMail.zimbra@redhat.com> In-Reply-To: <725955967.32271042.1454606645414.JavaMail.zimbra@redhat.com> References: <1453465198-11000-1-git-send-email-lprosek@redhat.com> <20160203123639.GA20527@grmbl.mre> <56B24A9C.9010104@redhat.com> <725955967.32271042.1454606645414.JavaMail.zimbra@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH] rng-random: implement request queue List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini Cc: Amit Shah , pagupta@redhat.com, qemu-devel@nongnu.org ----- 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 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 > > > > > >