From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:45161) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dAWrs-0004j0-0M for qemu-devel@nongnu.org; Tue, 16 May 2017 03:26:05 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dAWrm-0005mr-WA for qemu-devel@nongnu.org; Tue, 16 May 2017 03:26:03 -0400 Received: from mx1.redhat.com ([209.132.183.28]:60904) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1dAWrm-0005ln-Mf for qemu-devel@nongnu.org; Tue, 16 May 2017 03:25:58 -0400 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id B343B80464 for ; Tue, 16 May 2017 07:25:57 +0000 (UTC) From: Markus Armbruster References: <20170511163228.6666-1-quintela@redhat.com> <20170511163228.6666-3-quintela@redhat.com> <20170512034033.GN28293@pxdev.xzpeter.org> <877f1mgx9h.fsf@secure.mitica> <5bab598f-30eb-fcf6-9d06-8f683b466414@redhat.com> <87lgpyfo28.fsf@secure.mitica> <87k25iqf4m.fsf@dusky.pond.sub.org> <87efvqdqqj.fsf@secure.mitica> Date: Tue, 16 May 2017 09:25:54 +0200 In-Reply-To: <87efvqdqqj.fsf@secure.mitica> (Juan Quintela's message of "Mon, 15 May 2017 18:33:40 +0200") Message-ID: <871srpjm9p.fsf@dusky.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain Subject: Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Juan Quintela Cc: lvivier@redhat.com, qemu-devel@nongnu.org, Peter Xu , dgilbert@redhat.com Juan Quintela writes: > Markus Armbruster wrote: >> Juan Quintela writes: >> >>> Eric Blake wrote: > > >>>> Or is the proposal that we are also going to simplify the QMP 'migrate' >>>> command to get rid of crufty parameters? >>> >>> I didn't read it that way, but I would not oppose O:-) >>> >>> Later, Juan. >> >> I'm not too familiar with this stuff, so please correct my >> misunderstandings. >> >> "Normal" migration configuration is global state, i.e. it applies to all >> future migrations. >> >> Except the "migrate" command's flags apply to just the migration kicked >> off by that command. >> >> QMP command "migrate" has two flags "blk" (HMP: -b) and "inc" (HMP: -i). >> !blk && inc makes no sense and is silently treated like !blk && !inc. >> >> There's a third flag "detach" (HMP: -d), but it does nothing in QMP. > > As qmp command is asynchronous, you can think that -d is *always* on in > QMP O:-) Yes. The existence of "detach" in QMP is owed to limitations of early QMP infrastructure. It's flagged as "invalid" and "should not be used" since 2010. Perhaps we should start a section on QMP in . But I'd like to first have a way to communicate "you're using a deprecated feature" warnings via QMP. >> You'd like to deprecate these flags in favour of "normal" configuration. >> However, we need to maintain QMP backward compatibility at least for a >> while. HMP backward compatibility is nice to have, but not required. >> >> First step is to design the new interface you want. Second step is to >> figure out backward compatibility. >> >> The new interface adds a block migration tri-state (off, >> non-incremental, incremental) to global state, default off. Whether >> it's done as two bools or an enum of three values doesn't matter here. > > Tristates will complicate it. I still think that: > > - capability: block_migration > - parameter: block_shared > > Makes more sense, no? > > If block_migration is not enabled, we ignore the shared parameter. We > already do that for other parameters. My impression as a superficial reader is that migration configuration is a historically grown mess. Perhaps we shouldn't try to interpret too much intent into it :) If we redo migration as an instance of the "job" abstraction once we have it, then migration configuration & control should become more less messy. Of course, the old messes will stay with us for a while in the form of backward compatibility messes. I'm not too particular on how we do the tri-state now, as long as it reasonably fits what we have, and is documented clearly. >> If the new interface isn't used, the old one still needs to work. If it >> is used, the old one either has to do "the right thing", or fail >> cleanly. >> >> We approximate "new interface isn't used" by "block migration is off in >> global state". When it is off, the migration command needs to honor its >> two flags for compatibility. It must leave block migration off in >> global state. Yes, this will complicate the implementation until we >> actually remove the deprecated flags. Par for the backward compatility >> course. >> >> When block migration isn't off in global state, we can either >> >> * let the flags take precedence over the global state (one >> interpretation of "do the right thing"), or >> >> * reject flags that conflict with global state (another interpretation), >> or >> >> * reject *all* flags (fail cleanly). >> >> The last one looks perfectly servicable to me. > > Yeap, I think that makes sense. If you use capabilities, parameters, > old interface don't work at all. > > We still have a problem that is what happens if the user does: > > migrate -b > migrate_cancel (or error) > migrate (without -b) > > With current patches, it will still use -b. Fixing it requires still > anding more code. But I think that this use case is so weird what we > should not even care about it. It's a compatibility break. Whether it's tolerable is a judgement call, and not for me to make. Compatibility breaks need documentation, including release notes. Say you run migrate with -b by accident (say by recalling a prior command from persistent command history, such as qmp-shell's or rlwrap's or socat READLINE's), immediately realize what you've done and cancel the migration. Are you then stuck with -b forever?