From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1NCvYb-0003ez-N5 for qemu-devel@nongnu.org; Tue, 24 Nov 2009 08:35:50 -0500 Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1NCvYW-0003cR-ER for qemu-devel@nongnu.org; Tue, 24 Nov 2009 08:35:48 -0500 Received: from [199.232.76.173] (port=59794 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1NCvYV-0003cF-L0 for qemu-devel@nongnu.org; Tue, 24 Nov 2009 08:35:43 -0500 Received: from mx20.gnu.org ([199.232.41.8]:15154) by monty-python.gnu.org with esmtps (TLS-1.0:RSA_AES_256_CBC_SHA1:32) (Exim 4.60) (envelope-from ) id 1NCvYV-0006bm-3F for qemu-devel@nongnu.org; Tue, 24 Nov 2009 08:35:43 -0500 Received: from mail.codesourcery.com ([38.113.113.100]) by mx20.gnu.org with esmtp (Exim 4.60) (envelope-from ) id 1NCvYR-00063F-80 for qemu-devel@nongnu.org; Tue, 24 Nov 2009 08:35:39 -0500 From: Paul Brook Subject: Re: [Qemu-devel] Re: Re: Live migration protocol, device features, ABIs and other beasts Date: Tue, 24 Nov 2009 13:35:35 +0000 References: <4B0952C9.9010803@redhat.com> <4B09F0CA.3060705@codemonkey.ws> <20091124131717.GD2405@redhat.com> In-Reply-To: <20091124131717.GD2405@redhat.com> MIME-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Message-Id: <200911241335.35334.paul@codesourcery.com> List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: Paolo Bonzini , "Michael S. Tsirkin" > But it's easy to support migration to old qemu just > by discarding the INTx state, and this is not > at all harder, or worse, than migrating from old qemu > to new one. Do we really care about migrating to older versions? Migrating to a new version (backward compatibility) I see the use, it allows people to do upgrades with minimal downtime. I have my reservations about how feasible this is long-term, but within a release series it's not too bad. However is migrating to an old version (forward compatibility) really a worthwhile thing to support? It sounds like the sort of thing that we're never really going to test properly, so will probably fail a good proportion of the time anyway. Reading in old state files is a whole lot easier (to write maintain, and stay sane) than producing state that is bug-compatible with previous versions. This feels something where the best answer all round is "Don't do that". Paul