From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:48824) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1abLSQ-0001uU-9A for qemu-devel@nongnu.org; Thu, 03 Mar 2016 00:05:51 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1abLSN-00060T-0h for qemu-devel@nongnu.org; Thu, 03 Mar 2016 00:05:50 -0500 Received: from mx1.redhat.com ([209.132.183.28]:45755) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1abLSM-00060P-Ro for qemu-devel@nongnu.org; Thu, 03 Mar 2016 00:05:46 -0500 Received: from int-mx09.intmail.prod.int.phx2.redhat.com (int-mx09.intmail.prod.int.phx2.redhat.com [10.5.11.22]) by mx1.redhat.com (Postfix) with ESMTPS id 78A97C00070E for ; Thu, 3 Mar 2016 05:05:46 +0000 (UTC) Date: Thu, 3 Mar 2016 10:35:38 +0530 From: Amit Shah Message-ID: <20160303050538.GD15443@grmbl.mre> 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> <65843372.32316615.1454609255745.JavaMail.zimbra@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <65843372.32316615.1454609255745.JavaMail.zimbra@redhat.com> Subject: Re: [Qemu-devel] [PATCH] rng-random: implement request queue List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Ladi Prosek Cc: Paolo Bonzini , qemu-devel@nongnu.org, pagupta@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 > 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