From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:57749) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WksfJ-0004pj-EZ for qemu-devel@nongnu.org; Thu, 15 May 2014 06:13:34 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1WksfE-0001HJ-5B for qemu-devel@nongnu.org; Thu, 15 May 2014 06:13:29 -0400 Received: from mx1.redhat.com ([209.132.183.28]:32847) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WksfD-0001H2-UD for qemu-devel@nongnu.org; Thu, 15 May 2014 06:13:24 -0400 Date: Thu, 15 May 2014 13:12:12 +0300 From: "Michael S. Tsirkin" Message-ID: <20140515101212.GA22512@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> <20140515120826.050cf6f8@bahia.local> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <20140515120826.050cf6f8@bahia.local> 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: Greg Kurz Cc: Kevin Wolf , Fam Zheng , Stefan Hajnoczi , Juan Quintela , Alexander Graf , qemu-devel@nongnu.org, Anthony Liguori , Amit Shah , Paolo Bonzini , Andreas =?iso-8859-1?Q?F=E4rber?= On Thu, May 15, 2014 at 12:08:26PM +0200, Greg Kurz wrote: > On Thu, 15 May 2014 11:20:18 +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. > >=20 > > 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 >=20 > Not sure I understand well... Do you suggest to stream the markers firs= t, > then the device, then the subsections ? And then there would be a way > we can have the subsections restored before the device ? >=20 > > >>>>> All users of virtio_load()/virtio_save() need to be patched bec= ause the > > >>>>> subsections are streamed AFTER the device itself. > >=20 > > IMO this is calling for inversion of control - i.e. let virtio device= s > > call generic load/save functions that then dispatch to device-specifi= c > > code and let us add common stuff in a central place without forgettin= g > > to add calls in some new device. > >=20 >=20 > That makes a lot of sense. >=20 > > >>>> Since all have the same fixup, I'm wondering if a new section ca= n be > > >>>> added to the virtio-bus itself, which gets propagated to all dev= ices > > >>>> upon load in the dest. > > >>> > > >>> This calls for a way for devices to inherit properties from the b= us, > > >>> which doesn't exist ATM. > > >>> Fine but let's not hold up this patchset because of this. > > >> > > >> No, only suggestion is to add a migration section in the bus, and = then > > >> it's easier to do this in the post-migrate functions for each devi= ce > > >> -- so only one new section gets introduced instead of all devices > > >> being modified to send a new subsection. > > >> > > >=20 > > > The main problem I see is that virtio sucks: as you see in patch 8,= we have > > > to be careful not to call vring or virtqueue stuff before the devic= e knows > > > its endianness or it breaks... I need to study how the virtio-bus g= ets > > > migrated to ensure the endian section is streamed before the device= s. > >=20 > > There is no ordering guarantee. The state needs to be migrated in the > > device or bus where it sits, if post-load processing is required; i.e= ., > > if it's in VirtIODevice then something like this series, if it were o= n > > VirtioBus exclusively (device asking bus for its endianness each time > > and does not do post-load stuff) then endianness could be migrated as= a > > new bus section. Not sure if that would help the "broken" state thoug= h? > >=20 >=20 > IIRW the "broken" state was proposed as a per-device property... >=20 > Fam, >=20 > Do you have plans about the "broken" property ? Is it still needed ? >=20 > > Would touch on Stefan's alias properties for anything but virtio-mmio. > >=20 >=20 > OMG... maybe I should hold on then. No need to wait imho. Can this be made even simpler - call this stuff from virtio_save/virtio_load? Why not? > > Regards, > > Andreas > >=20 >=20 > Thanks ! >=20 > --=20 > Gregory Kurz kurzgreg@fr.ibm.com > gkurz@linux.vnet.ibm.c= om > Software Engineer @ IBM/Meiosys http://www.ibm.com > Tel +33 (0)562 165 496 >=20 > "Anarchy is about taking complete responsibility for yourself." > Alan Moore.