From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:33268) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Z5B1i-0003iB-G4 for qemu-devel@nongnu.org; Wed, 17 Jun 2015 06:57:06 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Z5B1c-0003NP-UC for qemu-devel@nongnu.org; Wed, 17 Jun 2015 06:57:02 -0400 Received: from mx1.redhat.com ([209.132.183.28]:35080) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Z5B1c-0003N3-O3 for qemu-devel@nongnu.org; Wed, 17 Jun 2015 06:56:56 -0400 Received: from int-mx10.intmail.prod.int.phx2.redhat.com (int-mx10.intmail.prod.int.phx2.redhat.com [10.5.11.23]) by mx1.redhat.com (Postfix) with ESMTPS id 2D8B131ED1E for ; Wed, 17 Jun 2015 10:56:56 +0000 (UTC) Date: Wed, 17 Jun 2015 12:56:53 +0200 From: "Michael S. Tsirkin" Message-ID: <20150617125550-mutt-send-email-mst@redhat.com> References: <1434480849-23093-1-git-send-email-dgilbert@redhat.com> <558124BD.8010807@redhat.com> <20150617094829-mutt-send-email-mst@redhat.com> <55812BC4.3000600@redhat.com> <20150617121148-mutt-send-email-mst@redhat.com> <87381q7grj.fsf@neno.neno> <20150617105157.GD2122@work-vm> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150617105157.GD2122@work-vm> Subject: Re: [Qemu-devel] [PATCH] Migration compatibility for serial List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Dr. David Alan Gilbert" Cc: amit.shah@redhat.com, Paolo Bonzini , qemu-devel@nongnu.org, Juan Quintela On Wed, Jun 17, 2015 at 11:51:57AM +0100, Dr. David Alan Gilbert wrote: > * Juan Quintela (quintela@redhat.com) wrote: > > "Michael S. Tsirkin" wrote: > > > On Wed, Jun 17, 2015 at 10:11:48AM +0200, Paolo Bonzini wrote: > > >> > > >> > > >> On 17/06/2015 09:52, Michael S. Tsirkin wrote: > > >> > > No, please. Upstream QEMU doesn't want to get into judgement about when > > >> > > migration quality might be "good enough" that you can drop subsections. > > >> > > It's one thing to perfect the .needed functions to make the appearance > > >> > > of subsections as unlikely as possible, but adding flags is not > > >> > > something we've done so far---and not something at least *I* want to do. > > >> > > > >> > Not like this, sure. But e.g. patches that force specific fields to > > >> > behave in a way consistent with QEMU 2.2, with appropriate > > >> > doducmentation would be ok I think. > > >> > > >> That's not what 2.2 means in "pc-i440fx-2.2". It means "same hardware > > >> as 2.2", not "bug-compatible with 2.2". > > >> > > >> Refining the .needed functions (e.g. see commit bfa7362889) is just > > >> that: describing when a subsection is needed. Forcing specific fields > > >> to behave in a way consistent with QEMU 2.2 is bug compatibility. > > >> > > >> Paolo > > > > > > We do bug-compatible if it's not a big pain, too. > > > > In this case, there is disagreement about what is better: > > - correct solution > > - bug compatible > > > > We can't have both in this case :-( > > > > Notice that if "both" are 2.2 , i.e. 2.3 with -M > > pc-i440fx-2.2, we also got the correct behaviour. So the matrix is > > something like: > > > > Source: 2.2 Destination: 2.2 -> bug compatible 2.2 > > Source: 2.3 Destination: 2.2 -> breaks if serial is being used, works otherwise > > Source: 2.3 Destination: 2.3 with -M pc-i440fx-2.2: works always > > To be fair the 2.3->2.2 is more subtle; opening it is unlikely > to generate the subsections; it needs a bit more than that (certainly on Linux) > figuring out exactly what triggers each subsection is trickier. > > Dave And more importantly, what is the result of skipping them, like you proposed. E.g. if guests crash that's no better than failing migration. > > > > > > So the problem is 2.3 -> 2.2 when serial is being used (notice that just > > opening it it is using). That is what we are differing about what is > > the right thing to do. As Paolo says, in upstream, we have done in the > > past the correct thing, in downstream, it depends. > > > > Notice that adding this patch makes that the three cases are bug > > compatible, i.e. there is no way to detect breakage neither a way to fix > > the issue (fix without the patch is just upgrade both binaries. > > ) > > > > Later, Juan. > -- > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK