From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 13FD7C433EF for ; Thu, 30 Sep 2021 13:34:47 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 9B78261440 for ; Thu, 30 Sep 2021 13:34:46 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org 9B78261440 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=nongnu.org Received: from localhost ([::1]:33444 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1mVwCz-0002HP-Oa for qemu-devel@archiver.kernel.org; Thu, 30 Sep 2021 09:34:45 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:34516) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mVw2o-0007wl-Cu for qemu-devel@nongnu.org; Thu, 30 Sep 2021 09:24:14 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:37728) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mVw2j-0006HU-SM for qemu-devel@nongnu.org; Thu, 30 Sep 2021 09:24:14 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1633008249; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=54nE8b0iKrjH2vrTLA6gs5NLiF0jFuHmmpoE8WMIOqk=; b=TXmd2a1HGOPastiK8KplRUbkefBQqT1GCH1yJzjzVTVzMg5okjmqSVRwOkj79V0LO3n2rZ oNGhsPWHpnZ5YSiqmCk9izZqqjvsBk8BHFoDJ3ZkRSfpYRTxuy0sU7FnqDb72c3J5UDjVC iu2OsJhLRtLlcAVijuUDYFJWspYNhnA= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-14-cO2PBGbQNVWbwrVE7yh2LA-1; Thu, 30 Sep 2021 09:24:08 -0400 X-MC-Unique: cO2PBGbQNVWbwrVE7yh2LA-1 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 mimecast-mx01.redhat.com (Postfix) with ESMTPS id BB6991006AA2; Thu, 30 Sep 2021 13:24:06 +0000 (UTC) Received: from localhost.localdomain.com (unknown [10.39.195.104]) by smtp.corp.redhat.com (Postfix) with ESMTP id 1AA7F5D9CA; Thu, 30 Sep 2021 13:24:02 +0000 (UTC) From: =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= To: qemu-devel@nongnu.org Subject: [PATCH v3 03/19] docs/devel: document expectations for QAPI data modelling for QMP Date: Thu, 30 Sep 2021 14:23:33 +0100 Message-Id: <20210930132349.3601823-4-berrange@redhat.com> In-Reply-To: <20210930132349.3601823-1-berrange@redhat.com> References: <20210930132349.3601823-1-berrange@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=berrange@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Laurent Vivier , Thomas Huth , =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= , Eduardo Habkost , David Hildenbrand , Michael Roth , Cornelia Huck , Yuval Shaia , Markus Armbruster , Peter Xu , "Dr. David Alan Gilbert" , Halil Pasic , Christian Borntraeger , qemu-s390x@nongnu.org, Gerd Hoffmann , Paolo Bonzini , Richard Henderson , Eric Blake Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Traditionally we have required that newly added QMP commands will model any returned data using fine grained QAPI types. This is good for commands that are intended to be consumed by machines, where clear data representation is very important. Commands that don't satisfy this have generally been added to HMP only. In effect the decision of whether to add a new command to QMP vs HMP has been used as a proxy for the decision of whether the cost of designing a fine grained QAPI type is justified by the potential benefits. As a result the commands present in QMP and HMP are non-overlapping sets, although HMP comamnds can be accessed indirectly via the QMP command 'human-monitor-command'. One of the downsides of 'human-monitor-command' is that the QEMU monitor APIs remain tied into various internal parts of the QEMU code. For example any exclusively HMP command will need to use 'monitor_printf' to get data out. It would be desirable to be able to fully isolate the monitor implementation from QEMU internals, however, this is only possible if all commands are exclusively based on QAPI with direct QMP exposure. The way to achieve this desired end goal is to finese the requirements for QMP command design. For cases where the output of a command is only intended for human consumption, it is reasonable to want to simplify the implementation by returning a plain string containing formatted data instead of designing a fine grained QAPI data type. This can be permitted if-and-only-if the command is exposed under the 'x-' name prefix. This indicates that the command data format is liable to future change and that it is not following QAPI design best practice. The poster child example for this would be the 'info registers' HMP command which returns printf formatted data representing CPU state. This information varies enourmously across target architectures and changes relatively frequently as new CPU features are implemented. It is there as debugging data for human operators, and any machine usage would treat it as an opaque blob. It is thus reasonable to expose this in QMP as 'x-query-registers' returning a 'str' field. Reviewed-by: Markus Armbruster Signed-off-by: Daniel P. Berrangé --- docs/devel/writing-monitor-commands.rst | 27 +++++++++++++++++++++++++ 1 file changed, 27 insertions(+) diff --git a/docs/devel/writing-monitor-commands.rst b/docs/devel/writing-monitor-commands.rst index a973c48f66..0f3b751dab 100644 --- a/docs/devel/writing-monitor-commands.rst +++ b/docs/devel/writing-monitor-commands.rst @@ -350,6 +350,33 @@ In this section we will focus on user defined types. Please, check the QAPI documentation for information about the other types. +Modelling data in QAPI +~~~~~~~~~~~~~~~~~~~~~~ + +For a QMP command that to be considered stable and supported long term, +there is a requirement returned data should be explicitly modelled +using fine-grained QAPI types. As a general guide, a caller of the QMP +command should never need to parse individual returned data fields. If +a field appears to need parsing, then it should be split into separate +fields corresponding to each distinct data item. This should be the +common case for any new QMP command that is intended to be used by +machines, as opposed to exclusively human operators. + +Some QMP commands, however, are only intended as ad hoc debugging aids +for human operators. While they may return large amounts of formatted +data, it is not expected that machines will need to parse the result. +The overhead of defining a fine grained QAPI type for the data may not +be justified by the potential benefit. In such cases, it is permitted +to have a command return a simple string that contains formatted data, +however, it is mandatory for the command to use the 'x-' name prefix. +This indicates that the command is not guaranteed to be long term +stable / liable to change in future and is not following QAPI design +best practices. An example where this approach is taken is the QMP +command "x-query-registers". This returns a formatted dump of the +architecture specific CPU state. The way the data is formatted varies +across QEMU targets, is liable to change over time, and is only +intended to be consumed as an opaque string by machines. + User Defined Types ~~~~~~~~~~~~~~~~~~ -- 2.31.1