From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:52841) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WksQw-0004P1-8V for qemu-devel@nongnu.org; Thu, 15 May 2014 05:58:51 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1WksQm-0004sB-TV for qemu-devel@nongnu.org; Thu, 15 May 2014 05:58:38 -0400 Received: from cantor2.suse.de ([195.135.220.15]:40028 helo=mx2.suse.de) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WksQm-0004qD-Np for qemu-devel@nongnu.org; Thu, 15 May 2014 05:58:28 -0400 Message-ID: <53748FC1.5010702@suse.de> Date: Thu, 15 May 2014 11:58:25 +0200 From: =?ISO-8859-1?Q?Andreas_F=E4rber?= MIME-Version: 1.0 References: <20140514154130.10746.1412.stgit@bahia.local> <20140514154137.10746.94708.stgit@bahia.local> <20140515060425.GA31192@grmbl.mre> <20140515062351.GB14456@redhat.com> <20140515064635.GB31192@grmbl.mre> <20140515090449.2db0cbe0@bahia.local> <537486D2.1060609@suse.de> <20140515095208.GB16405@redhat.com> In-Reply-To: <20140515095208.GB16405@redhat.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [PATCH RFC 1/8] virtio: add subsections to the migration stream List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Michael S. Tsirkin" Cc: Kevin Wolf , Fam Zheng , Stefan Hajnoczi , Juan Quintela , Alexander Graf , qemu-devel@nongnu.org, Anthony Liguori , Amit Shah , Paolo Bonzini , Greg Kurz Am 15.05.2014 11:52, schrieb Michael S. Tsirkin: > On Thu, May 15, 2014 at 11:20:18AM +0200, Andreas F=E4rber wrote: >> Am 15.05.2014 09:04, schrieb Greg Kurz: >>> On Thu, 15 May 2014 12:16:35 +0530 >>> Amit Shah wrote: >>>> On (Thu) 15 May 2014 [09:23:51], Michael S. Tsirkin wrote: >>>>> On Thu, May 15, 2014 at 11:34:25AM +0530, Amit Shah wrote: >>>>>> On (Wed) 14 May 2014 [17:41:38], Greg Kurz wrote: >>>>>>> Since each virtio device is streamed in its own section, the idea= is to >>>>>>> stream subsections between the end of the device section and the = start >>>>>>> of the next sections. This allows an older QEMU to complain and e= xit >>>>>>> when fed with subsections: >>>>>>> >>>>>>> Unknown savevm section type 5 >>>>>>> Error -22 while loading VM state >>>>>> >>>>>> Please make this configurable -- either via configure or device >>>>>> properties. That avoids having to break existing configurations t= hat >>>>>> work without this patch. >> >> Since backwards migration is not supported upstream, wouldn't it be >> easiest to just add support for the subsection marker and skipping to >> the end of section in that downstream? >=20 > Backwards and forwards migration need to be supported, > customers told us repeatedly. So some downstreams support this > and not supporting it upstream just means downstreams need > to do their own thing. >=20 > As importantly, ping-pong migration is the only > reliable way to stress migration. >=20 > So if we want to test cross-version we need it to work > both way. >=20 > Finally, the real issue and difficulty with cross-version migration is > making VM behave in a backwards compatible way. Serializing in a > compatible way is a trivial problem, or would be if the code wasn't a > mess :) Once you do the hard part, breaking migration because of the > trivial serialization issue is just silly. And special-casing forward > migration does not make code simpler, it really only leads to > proliferation of hacks and lack of symmetry. >=20 > So yes it's a useful feature, and no not supporting it does > not help anyway. It seems you misunderstand. I was not saying it's not useful. My point is that VMStateSubsections added in newer versions (and thus not existing in older versions) need to be handled for any VMState-converted devices. So why can't we make that work for virtio too? Andreas --=20 SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 N=FCrnberg, Germany GF: Jeff Hawn, Jennifer Guild, Felix Imend=F6rffer; HRB 16746 AG N=FCrnbe= rg