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 CC20AC433F5 for ; Sat, 6 Nov 2021 06:37:07 +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 18E4060ED5 for ; Sat, 6 Nov 2021 06:37:07 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org 18E4060ED5 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]:48540 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1mjFK6-0004nK-4R for qemu-devel@archiver.kernel.org; Sat, 06 Nov 2021 02:37:06 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:47206) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mjFHu-0003zW-P4 for qemu-devel@nongnu.org; Sat, 06 Nov 2021 02:34:50 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:44623) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mjFHq-0005Su-5U for qemu-devel@nongnu.org; Sat, 06 Nov 2021 02:34:49 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1636180481; 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: in-reply-to:in-reply-to:references:references; bh=QCDLt3yLQn0Se/nRUEWbK6sbAeYrVZmMf3q/Xvon9mQ=; b=G8/xRGBYURWIfrgs+MFr/pgAGwI5JgBSHCVgzmt808bum+MvYoGlo5MPUeIPpLtO5gQiFt vJXfU3nXs8QLS3j8ioUVGjkavst1A/eAjtmSLLJxQAfzVZuf/ImYniseaFh1NCQr5esAxc lr1WUb4G0dWmbM8LraNBfFVh3mqh97A= 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-463-ARUM4-0bMWae--iYlDWRpA-1; Sat, 06 Nov 2021 02:34:38 -0400 X-MC-Unique: ARUM4-0bMWae--iYlDWRpA-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 61AFC1808319; Sat, 6 Nov 2021 06:34:36 +0000 (UTC) Received: from blackfin.pond.sub.org (ovpn-112-7.ams2.redhat.com [10.36.112.7]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 96D0D60C17; Sat, 6 Nov 2021 06:34:26 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 265C911380A7; Sat, 6 Nov 2021 07:34:25 +0100 (CET) From: Markus Armbruster To: Kevin Wolf Subject: Re: [PATCH v3 03/12] vfio-user: define vfio-user-server object References: <13dba991f1de91711e5c3cad9a332d6e7c5eee7b.1633929457.git.jag.raman@oracle.com> <6346833B-469B-487B-8382-62EA03BA56C2@oracle.com> <87wnloce5t.fsf@dusky.pond.sub.org> <87y2623of6.fsf@dusky.pond.sub.org> Date: Sat, 06 Nov 2021 07:34:25 +0100 In-Reply-To: (Kevin Wolf's message of "Fri, 5 Nov 2021 14:19:34 +0100") Message-ID: <87fss9vlla.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 2.79 on 10.5.11.12 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 Received-SPF: pass client-ip=216.205.24.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.735, 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: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Elena Ufimtseva , John Johnson , "thuth@redhat.com" , pkrempa@redhat.com, Jag Raman , "swapnil.ingle@nutanix.com" , "john.levon@nutanix.com" , "alex.bennee@linaro.org" , qemu-devel , Alex Williamson , =?utf-8?Q?Marc-Andr=C3=A9?= Lureau , Stefan Hajnoczi , "thanos.makatos@nutanix.com" , "pbonzini@redhat.com" , "philmd@redhat.com" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Kevin Wolf writes: > Am 05.11.2021 um 11:08 hat Markus Armbruster geschrieben: >> Kevin Wolf writes: >> >> > Am 04.11.2021 um 13:13 hat Markus Armbruster geschrieben: >> >> The old syntax almost always has its quirks. Ideally, we'd somehow get >> >> from quirky old to boring new in an orderly manner. Sadly, we still >> >> don't have good solutions for that. To make progress, we commonly >> >> combine JSON new with quirky old. >> >> >> >> qemu-system-FOO -object works that way. object_option_parse() parses >> >> either JSON or QemuOpts. It wraps the former in a QObject visitor, and >> >> the latter in an opts visitor. >> >> >> >> QemuOpts is flat by design[*], so the opts visitor parses flat QemuOpts >> >> from a (possibly non-flat) QAPI type. How exactly it flattens, and how >> >> it handles clashes I don't remember. >> >> >> >> Sadly, this means that we get quirky old even for new object types. >> > >> > For -object in the system emulator (the tools all use the keyval >> > visitor, so there it would work as expected), the only reason that we >> > need to keep the quirky old code path around is the list handling in >> > memory-backend.host-nodes. >> > >> > The main difficulty there is that the old QemuOpts based code path >> > allows specifying the option twice and both of them would effectively be >> > combined. Do we have any idea how to replicate this in a keyval parser >> > based world? >> >> I can see just two clean solutions, but both involve upending a lot of >> code. >> >> We can fuse keyval parser and visitor to get a schema-directed parser. >> >> We can change the abstract keyval syntax to permit repeated keys. This >> means replacing QDict in in the abstract syntax tree, with fallout in >> the visitor. >> >> Even if we find a practical solution, I don't like the combination of >> "you may give the same parameter multiple times, and the last one wins" >> and "for a list-valued parameter, the values of repeated parameters are >> collected into a list". Each makes sense on its own. The combination >> not so much. Inheriting "last one wins" from QemuOpts may have been a >> mistake. >> >> The keyval way of doing lists (inherited from the block layer's usage of >> dotted keys? I don't remember) requires the user to count, which isn't >> exactly nice, either. > > Yes. If we didn't have to maintain compatibility (or actually as soon as > we degrade non-JSON option lists to HMP-level support), I would > introduce [] and {} syntax for lists and dicts, even if that means that > use of these characters in strings doesn't work any more or only in a > limited way. I think this would be the best compromise for usability. > > Anyway, this doesn't help us with the compatibility problem we're > discussing here. > >> > If not, do we want to use the remaining time until 6.2 to deprecate >> > this? The nasty part is that the only syntax that works both now and in >> > the future is JSON. We can't easily accept the new keyval syntax while >> > still using the QemuOpts based code. >> >> What exactly do you propose to deprecate? > > We can deprecate on two different levels. I think it's useful to do > both: > > 1. Broad deprecation: Stable non-JSON interfaces are degraded to > a HMP-like compatibility promise. Calling it "deprecation" might be confusing. HMP isn't deprecated, it's merely not a stable interface. That's kind of like "deprecated when you need stable", but saying "not a stable interface" is clearer. When I write "deprecate" below, I mean something like "go use something else (no conditions)". When I mean "use something else when you need stable", I write "degrade" (short for "degrade to an HMP-like compatibility promise"). > Obviously, this can only be done > for options that support JSON. We can also degrade or even deprecate sugar options in favor of the real ones. Case by case, I guess. > Peter Maydell also wants to do this > only after a big user (read: libvirt) has implemented and is > using JSON, basically as a proof that the alternative is working. > > So this can certainly be done for -object. I believe libvirt also > uses JSON for -device now, so this should be fine now, too. The non-sugar options supporting JSON are -audiodev, -blockdev, -compat, -display (partially), -machine (I think), -object. -netdev is QAPIfied, but still uses QemuOpts. Too late for 6.2, I'm afraid. > Possibly > -drive (in favour of -blockdev), though I'm not completely sure if we > have gotten rid of the final users of -drive. (CCing Peter Krempa for > details.) The problem with deprecating -drive is configuring onboard block devices. We need a stable interface for that, and it must be usable together with -blockdev. We provided such an interface (machine properties) for some onboard block devices starting with commit ebc29e1bea "pc: Support firmware configuration with -blockdev". Many more remain, I believe. > This degradation of the compatibility promise doesn't tell users what > exactly is going to change, which is why doing the second one, too, > might be nice. > > 2. Narrow deprecation: We can just deprecate the non-JSON form, or > certain aspects of it, of memory-backend.host-nodes. This is the > specific things that stops us from switching -object to keyval. > > a. Deprecate the whole option. If you want to use it and need a > stable interface, you have to use JSON. We'll just switch the > non-JSON form on a flag day. Before it, you need to use QemuOpts + > OptsVisitor syntax for the list; after it, you need to use keyval > syntax. I parse "the whole option" as "-object with dotted keys argument". Correct? > b. Deprecate only repeating the option. memory-backend is changed to > first try visiting a list, and if that fails, it visits a string > and goes through a string visitor locally to keep supporting the > integer range syntax. Possible problem: integer range syntax must not leak into the JSON form. > c. Deprecate all list values, but keep supporting a single integer > value by using an alternate between list and int. Single int should also not leak into JSON. > Picking one of these four options is enough to convert -object to > keyval. I would suggest doing both 1. and one of the options in 2. I'm grateful for your analysis.