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 X-Spam-Level: X-Spam-Status: No, score=-13.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5924BC433F5 for ; Thu, 9 Sep 2021 09:35:05 +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 AB3F860E0B for ; Thu, 9 Sep 2021 09:35:04 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org AB3F860E0B 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]:44570 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1mOGSV-0008Q5-Up for qemu-devel@archiver.kernel.org; Thu, 09 Sep 2021 05:35:03 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:46184) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mOGQz-0006On-D5 for qemu-devel@nongnu.org; Thu, 09 Sep 2021 05:33:29 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:42666) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mOGQx-0001BH-7w for qemu-devel@nongnu.org; Thu, 09 Sep 2021 05:33:28 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1631180006; 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=/7I22n1JlMBc0Gpi1UDzpt+2MYrFBUjfo6McnPWUPxY=; b=D9kTSU/vPGI5eP1Lb62v1QZQJqcQjCJBKUutaKPzMgBrLTgwAt3NPldymbC5FWlrmBy2Uq 5m9GDolUSiwVZhynaU22/Mw1MSSJK17AxzeMyg7cEnRBs+DW5cN+c8N/+VBBAMsBDazop5 Es6wXqkUsk9+3qnMSo+Qr+86Wpi2dVE= 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-34-weNaCH4FN72kTbEb4bh6zw-1; Thu, 09 Sep 2021 05:33:23 -0400 X-MC-Unique: weNaCH4FN72kTbEb4bh6zw-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id BEDA180006E; Thu, 9 Sep 2021 09:33:22 +0000 (UTC) Received: from blackfin.pond.sub.org (ovpn-112-14.ams2.redhat.com [10.36.112.14]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 67A3F19C79; Thu, 9 Sep 2021 09:33:22 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id EA4741138606; Thu, 9 Sep 2021 11:33:20 +0200 (CEST) From: Markus Armbruster To: Daniel P. =?utf-8?Q?Berrang=C3=A9?= Subject: Re: [PATCH 1/5] docs/devel: document expectations for QAPI data modelling for QMP References: <20210908103711.683940-1-berrange@redhat.com> <20210908103711.683940-2-berrange@redhat.com> Date: Thu, 09 Sep 2021 11:33:20 +0200 In-Reply-To: <20210908103711.683940-2-berrange@redhat.com> ("Daniel P. =?utf-8?Q?Berrang=C3=A9=22's?= message of "Wed, 8 Sep 2021 11:37:07 +0100") Message-ID: <87tuium6u7.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.2 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=armbru@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Received-SPF: pass client-ip=170.10.133.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -31 X-Spam_score: -3.2 X-Spam_bar: --- X-Spam_report: (-3.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.393, 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: Eduardo Habkost , Eric Blake , qemu-devel@nongnu.org, "Dr. David Alan Gilbert" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Daniel P. Berrang=C3=A9 writes: > 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. > > Signed-off-by: Daniel P. Berrang=C3=A9 > --- > docs/devel/writing-qmp-commands.rst | 25 +++++++++++++++++++++++++ > 1 file changed, 25 insertions(+) > > diff --git a/docs/devel/writing-qmp-commands.rst b/docs/devel/writing-qmp= -commands.rst > index 6a10a06c48..d032daa62d 100644 > --- a/docs/devel/writing-qmp-commands.rst > +++ b/docs/devel/writing-qmp-commands.rst > @@ -350,6 +350,31 @@ In this section we will focus on user defined types.= Please, check the QAPI > documentation for information about the other types. > =20 > =20 > +Modelling data in QAPI > +~~~~~~~~~~~~~~~~~~~~~~ Outline now: How to write QMP commands using the QAPI framework Overview Testing Writing a command that doesn't return data Arguments Errors Command Documentation Implementing the HMP command Writing a command that returns data --> Modelling data in QAPI User Defined Types The HMP command Returning Lists Awkward. I guess you wanted it next to "User Defined Types", which makes some sense. Perhaps minor tweaks (headlines, maybe a bit of text as well) would suffice: How to write QMP commands using the QAPI framework Overview Testing Writing a simple command: hello-world Arguments Errors Command Documentation Implementing the HMP command Writing more complex commands Modelling data in QAPI User Defined Types The HMP command Returning Lists > + > +For a QMP command that to be considered stable and supported long term t= here "term, there" > +is a requirement returned data should be explicitly modelled using fine = grained "fine-grained", I think. > +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 par= sing, > +them it should be split into separate fields corresponding to each disti= nct > +data item. This should be the common case for any new QMP command that i= s > +intended to be used by machines, as opposed to exclusively human operato= rs. > + > +Some QMP commands, however, are only intended as adhoc debugging aids fo= r 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 de= fining > +a fine grained QAPI type for the data may not be justified by the potent= ial > +benefit. In such cases, it is permitted to have a command return a simpl= e string There are many existing long lines in this file, so I'm not flagging yours, except for this one, because it increases the maximum. > +that contains formatted data, however, it is mandatory for the command t= o 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 comma= nd > +"x-query-registers". This returns a printf formatted dump of the archite= cture Drop "printf". > +specific CPU state. The way the data is formatted varies across QEMU tar= gets, > +is liable to change over time, and is only intended to be consumed as an= opaque > +string by machines. > + > User Defined Types > ~~~~~~~~~~~~~~~~~~ This file is a tutorial. It teaches command writing by examples[*]. I think it should also teach this new class of "ad hoc" QMP commands the same way. A section "Writing a debugging aid returning unstructured text" could go at the very end. [*] Bit-rotten in places, but that's life for docs.