From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:41490) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1co6CG-0002Gf-5G for qemu-devel@nongnu.org; Wed, 15 Mar 2017 06:30:25 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1co6CC-0003R1-7a for qemu-devel@nongnu.org; Wed, 15 Mar 2017 06:30:24 -0400 Received: from mx1.redhat.com ([209.132.183.28]:37428) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1co6CC-0003Qf-2H for qemu-devel@nongnu.org; Wed, 15 Mar 2017 06:30:20 -0400 Received: from int-mx10.intmail.prod.int.phx2.redhat.com (int-mx10.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 0E43DC057FA6 for ; Wed, 15 Mar 2017 10:30:20 +0000 (UTC) From: Markus Armbruster References: <20170301123223.12489-1-berrange@redhat.com> <20170302123300.GJ29835@redhat.com> <87d1dys9mr.fsf@dusky.pond.sub.org> <20170314184909.GC7576@work-vm> <87bmt3t5r0.fsf@dusky.pond.sub.org> <20170315093226.GB2430@work-vm> Date: Wed, 15 Mar 2017 11:30:17 +0100 In-Reply-To: <20170315093226.GB2430@work-vm> (David Alan Gilbert's message of "Wed, 15 Mar 2017 09:32:26 +0000") Message-ID: <87h92ultmu.fsf@dusky.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain Subject: Re: [Qemu-devel] [PATCH] migration: allow clearing migration string parameters List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Dr. David Alan Gilbert" Cc: qemu-devel@nongnu.org, Juan Quintela "Dr. David Alan Gilbert" writes: > * Markus Armbruster (armbru@redhat.com) wrote: >> "Dr. David Alan Gilbert" writes: [...] >> > I'm confused why we need a 'reset to default' - all we need is the ability >> > to change each parameter, and for the new value of that parameter >> > to be an empty string. >> >> You argue syntax, I'm arguing semantics. >> >> The command means "set parameter P to value V". *Except* when V is "", >> it means something else, namely "reset parameter P to its default, >> whatever that may be". >> >> This is (a) not general, because it won't do for cases where "" may >> occur as value, and (b) ugly. >> >> Ugliness is the eye of the beholder. Lack of generality isn't. > > No, I'm questioning why it's defined as 'reset parameter P to its default'; > why do we need a way to do that? Dan's commit message explains: Some of the migration parameters are strings, which default to NULL, eg tls_hostname and tls_creds. The mgmt app will set the tls_creds parameter on both source and target QEMU instances, in order to trigger use of TLS for migration. After performing a TLS encrypted migration though, migration might be used for other reasons - for example, to save the QEMU state to a file. We need TLS turned off when doing this, but the migrate-set-parameters QAPI command does not provide any facility to clear/reset parameters to their default state. If you simply ommit the tls_creds parameter in migrate-set-parameters, then 'has_tls_creds' will be false and so no action will be taken. The only option that works with migrate-set-parameters is to treat "" on the wire as equivalent to requesting NULL. Failing that we would have to create a new 'migrate-reset-parameters' method to explicitly put a parameter back to its default value.