From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1Nc0vF-0003za-44 for qemu-devel@nongnu.org; Mon, 01 Feb 2010 13:22:53 -0500 Received: from [199.232.76.173] (port=53266 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1Nc0vE-0003zM-O7 for qemu-devel@nongnu.org; Mon, 01 Feb 2010 13:22:52 -0500 Received: from Debian-exim by monty-python.gnu.org with spam-scanned (Exim 4.60) (envelope-from ) id 1Nc0v7-00024x-U0 for qemu-devel@nongnu.org; Mon, 01 Feb 2010 13:22:47 -0500 Received: from mx1.redhat.com ([209.132.183.28]:20086) by monty-python.gnu.org with esmtp (Exim 4.60) (envelope-from ) id 1Nc0v7-00024g-EA for qemu-devel@nongnu.org; Mon, 01 Feb 2010 13:22:45 -0500 Received: from int-mx01.intmail.prod.int.phx2.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) by mx1.redhat.com (8.13.8/8.13.8) with ESMTP id o11IMi3e015678 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK) for ; Mon, 1 Feb 2010 13:22:44 -0500 Date: Mon, 1 Feb 2010 16:22:34 -0200 From: Luiz Capitulino Subject: Re: [Qemu-devel] [PATCH 0/8]: QMP feature negotiation support Message-ID: <20100201162234.0f144f3f@doriath> In-Reply-To: References: <1264686180-29845-1-git-send-email-lcapitulino@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Markus Armbruster Cc: qemu-devel@nongnu.org On Mon, 01 Feb 2010 18:08:27 +0100 Markus Armbruster wrote: > Luiz Capitulino writes: > > > Feature negotiation allows clients to enable new QMP capabilities they > > support and thus allows QMP to envolve in a compatible way. > > > > A capability is a new QMP feature and/or protocol change which is not part of > > the core protocol as defined in the QMP spec. > > Well, it becomes part of the protocol then. But I understand what you > mean. > > > Feature negotiation is implemented by, among other changes, adding > > mode-oriented support to QMP, which comprehends the following: > > > > o Two modes: handshake and operational > > o All QMP Monitors start in handshake mode > > o In handshake mode only commands to query/enable/disable QMP capabilities are > > allowed (there are few exceptions) > > o Clients can switch to the operational mode at any time > > o In Operational mode most commands are allowed and QMP capabilities changes > > made in handshake mode take effect > > > > Please, note that we don't have any capability yet. So, the most visable > > change in this series is that now Clients must switch to operational mode to > > be able to issue 'regular' commands. > > > > Session example: > > > > """ > > {"QMP": {"capabilities": []}} > > > > { "execute": "query-qmp-mode" } > > {"return": {"mode": "handshake"}} > > > > { "execute": "stop" } > > {"error": {"class": "CommandNotFound", "desc": "The command stop has not been found", "data": {"name": "stop"}}} > > > > { "execute": "qmp_capability_enable", "arguments": { "name": "foobar" } } > > {"error": {"class": "InvalidParameter", "desc": "Invalid parameter name", "data": {"name": "name"}}} > > > > { "execute": "qmp_switch_mode", "arguments": { "mode": "operational" } } > > {"return": {}} > > > > { "execute": "query-qmp-mode" } > > {"return": {"mode": "operational"}} > > > > { "execute": "stop" } > > {"return": {}} > > > > """ > > I don't doubt your design does the job. I just think it's overly > general. I had something far more stupid in mind: > > client connects > server -> client: version & capability offer (one message) > again: > client -> server: capability selection (one message) > server -> client: either okay or error (one message) > if error goto again > connection is now ready for commands > > No modes. The distinct lack of generality is a design feature. I like the simplicity and if we were allowed to change later I'd do it. The question is if we will ever want features to be _configured_ before the protocol is operational. In this case we'd need to pass feature arguments through the capability selection command, which will get ugly and hard to use/understand. Mode oriented support doesn't have this limitation. Maybe we won't never really use it, but it's safer.