From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:54718) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WksWa-0001xB-8w for qemu-devel@nongnu.org; Thu, 15 May 2014 06:04:34 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1WksWT-0006bA-Cs for qemu-devel@nongnu.org; Thu, 15 May 2014 06:04:28 -0400 Received: from mx1.redhat.com ([209.132.183.28]:35501) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WksWT-0006b3-5D for qemu-devel@nongnu.org; Thu, 15 May 2014 06:04:21 -0400 Date: Thu, 15 May 2014 13:03:09 +0300 From: "Michael S. Tsirkin" Message-ID: <20140515100309.GA22407@redhat.com> 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> <53748FC1.5010702@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <53748FC1.5010702@suse.de> 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: Andreas =?iso-8859-1?Q?F=E4rber?= Cc: Kevin Wolf , Fam Zheng , Stefan Hajnoczi , Juan Quintela , Alexander Graf , qemu-devel@nongnu.org, Anthony Liguori , Amit Shah , Paolo Bonzini , Greg Kurz On Thu, May 15, 2014 at 11:58:25AM +0200, Andreas F=E4rber wrote: > 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 id= ea is to > >>>>>>> stream subsections between the end of the device section and th= e start > >>>>>>> of the next sections. This allows an older QEMU to complain and= exit > >>>>>>> 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= that > >>>>>> 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 t= o > >> 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 i= s > > 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 forwar= d > > 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. >=20 > It seems you misunderstand. I was not saying it's not useful. >=20 > 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 to= o? >=20 > Andreas Sure. AFAIK the way to this works is by adding an "field_exists" callback, and not sending the section when we are in a compat mode. > --=20 > SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 N=FCrnberg, Germany > GF: Jeff Hawn, Jennifer Guild, Felix Imend=F6rffer; HRB 16746 AG N=FCrn= berg