From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:60602) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Z5B06-000180-34 for qemu-devel@nongnu.org; Wed, 17 Jun 2015 06:55:26 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Z5B00-0002UD-Ca for qemu-devel@nongnu.org; Wed, 17 Jun 2015 06:55:21 -0400 Received: from mx1.redhat.com ([209.132.183.28]:42411) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Z5B00-0002U4-8J for qemu-devel@nongnu.org; Wed, 17 Jun 2015 06:55:16 -0400 Received: from int-mx13.intmail.prod.int.phx2.redhat.com (int-mx13.intmail.prod.int.phx2.redhat.com [10.5.11.26]) by mx1.redhat.com (Postfix) with ESMTPS id CF8B52B51DB for ; Wed, 17 Jun 2015 10:55:15 +0000 (UTC) Date: Wed, 17 Jun 2015 12:55:12 +0200 From: "Michael S. Tsirkin" Message-ID: <20150617124852-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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87381q7grj.fsf@neno.neno> Subject: Re: [Qemu-devel] [PATCH] Migration compatibility for serial List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Juan Quintela Cc: amit.shah@redhat.com, Paolo Bonzini , "Dr. David Alan Gilbert (git)" , qemu-devel@nongnu.org On Wed, Jun 17, 2015 at 12:48:00PM +0200, Juan Quintela 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 :-( I think this depends on how major and/or common the bug is. > 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 > > > 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. I would say it's the "correct" thing. We don't really know for sure it's correct, we think so. We might in theory have bugs in 2.4 code too. Whether we are better off failing migration depends on how severe the bugs are. -- MST