From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:59167) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fn3ep-0004Sl-II for qemu-devel@nongnu.org; Tue, 07 Aug 2018 11:12:24 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fn3el-0005F8-Bq for qemu-devel@nongnu.org; Tue, 07 Aug 2018 11:12:23 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:58384 helo=mx1.redhat.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1fn3el-0005Ex-7k for qemu-devel@nongnu.org; Tue, 07 Aug 2018 11:12:19 -0400 Date: Tue, 7 Aug 2018 16:12:15 +0100 From: "Dr. David Alan Gilbert" Message-ID: <20180807151215.GM2556@work-vm> References: <20180807130355.29780-1-peter.maydell@linaro.org> <87muty2u57.fsf@trasno.org> <87in4m2qni.fsf@trasno.org> <87wot21bbt.fsf@trasno.org> <87o9ee1aka.fsf@trasno.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Subject: Re: [Qemu-devel] [PATCH] migration: Correctly handle subsections with no 'needed' function List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Peter Maydell Cc: Juan Quintela , QEMU Developers , "patches@linaro.org" * Peter Maydell (peter.maydell@linaro.org) wrote: > On 7 August 2018 at 16:05, Juan Quintela wrote: > > Peter Maydell wrote: > >> So it's better to use a needed-always subsection, which will give > >> the desired behaviour: > >> * new QEMU -> new QEMU: state is always migrated > >> * old QEMU -> new QEMU: migration doesn't fail, and guest > >> will work assuming it didn't care about this corner of the > >> device's functionality (the device will end up with state > >> as it was at reset, or possibly special-cased via > >> pre_load/post_load hooks for the "section not present" case) > > > > we break by definiton new QEMU -M into old QEMU. > > Is that supposed to work? I always thought that we never > supported migration back to an older QEMU version like that. Downstream we do a lot of hard work to keep that working for versioned machine types; using subsections makes this a lot easier and if it's done right in the first place then it works upstream as well. > In any case, for all the devices here the machine types are > not versioned. Right. Dave > thanks > -- PMM -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK