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 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 smtp.lore.kernel.org (Postfix) with ESMTPS id 42624C41513 for ; Wed, 17 Jul 2024 10:47:31 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1sU2BN-0003MO-NF; Wed, 17 Jul 2024 06:46:50 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1sU2BI-0003JH-LB for qemu-devel@nongnu.org; Wed, 17 Jul 2024 06:46:44 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1sU2BG-0000nY-C9 for qemu-devel@nongnu.org; Wed, 17 Jul 2024 06:46:44 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1721213200; h=from:from:reply-to: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=vqBPmWQlBfUWntYTUaw49ExwtwQuR2noQAup7PlxIw0=; b=OsO2X9SXdYEmetiMPe3TXgY192afAmFFXXQYNqBvn32ngVsVmr+jlp5eoBmTA8FWAOlEd5 0BP3NVqTr5MwmLiJbkQqnjO+Luud3+YMbyfW2Wv+/R6gPLa19pj+WQbi1aRnpBrOVNOVGy W2aS4f0tQZS1gtDVMFe+3rNJe2WLVCY= Received: from mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-90-ZPY91qMhPfikHSlC37nvPQ-1; Wed, 17 Jul 2024 06:46:38 -0400 X-MC-Unique: ZPY91qMhPfikHSlC37nvPQ-1 Received: from mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.40]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 048761955BEF; Wed, 17 Jul 2024 10:46:38 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.53]) by mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 618281955D42; Wed, 17 Jul 2024 10:46:36 +0000 (UTC) Date: Wed, 17 Jul 2024 11:46:32 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Markus Armbruster Cc: qemu-devel@nongnu.org, Michael Roth , Konstantin Kostiuk Subject: Re: [PATCH 04/14] qapi: add a 'command-features' pragma Message-ID: References: <20240604153242.251334-1-berrange@redhat.com> <20240604153242.251334-5-berrange@redhat.com> <87r0bzuj7d.fsf@pond.sub.org> <87a5inuh75.fsf@pond.sub.org> <87bk2xjjkl.fsf@pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <87bk2xjjkl.fsf@pond.sub.org> User-Agent: Mutt/2.2.12 (2023-09-09) X-Scanned-By: MIMEDefang 3.0 on 10.30.177.40 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: 12 X-Spam_score: 1.2 X-Spam_bar: + X-Spam_report: (1.2 / 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_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_SBL_CSS=3.335, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=no 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: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Tue, Jul 16, 2024 at 08:08:42PM +0200, Markus Armbruster wrote: > Sorry for the delay; too many distractions, and I needed a good think. > > Daniel P. Berrangé writes: > > > On Fri, Jul 12, 2024 at 10:50:54AM +0200, Markus Armbruster wrote: > >> Daniel P. Berrangé writes: > >> > >> > On Fri, Jul 12, 2024 at 10:07:34AM +0200, Markus Armbruster wrote: > >> >> Daniel P. Berrangé writes: > >> >> > >> >> > The 'command-features' pragma allows for defining additional > >> >> > special features that are unique to a particular QAPI schema > >> >> > instance and its implementation. > >> >> > >> >> So far, we have special features (predefined, known to the generator and > >> >> treated specially), and normal features (user-defined, not known to the > >> >> generator). You create a new kind in between: user-defined, not known > >> >> to the generator, yet treated specially (I guess?). Hmm. > >> >> > >> >> Could you at least hint at indented use here? What special treatment do > >> >> you have in mind? > >> > > >> > Essentially, these features are a way to attach metadata to commands that > >> > the server side impl can later query. This eliminates the need to hardcode > >> > lists of commands, such as in QGA which hardcodes a list of commands which > >> > are safe to use when filesystems are frozen. This is illustrated later in > >> > this series. > >> > >> Please update docs/devel/qapi-code-gen.rst section "Pragma directives", > >> and maybe section "Features". > > Second thoughts; see below. > > >> I'm not sure conflating the new kind of feature with existing special > >> features is a good idea. I need to review more of the series before I > >> can make up my mind. > > > > I originally implemented a completely separate 'tags' concept in the > > QAPI parser, before deciding I was just re-inventing 'features' for > > no obvious benefit. > > > > The other nice thing about using features is that these are exposed > > in the schema and docs. With the 'fsfreeze' restriction in code, > > there's no formal docs of what commands are allowed when frozen, and > > this is also not exposed in QAPI schema to apps. Using 'features' > > we get all that as standard. > > When you need to tack a mark to one or more things for whatever purpose > *and* expose it to QMP clients, then features make sense. This is the > case here. > > Initially, features were strictly an external interface annotation, and > were not meant to be used within QEMU. All features were user-defined. > > This changed when I created configurable policy for deprecated and > unstable management interfaces: the policy engine needs to check for > features 'deprecated' and 'unstable'. Since the policy engine is partly > implemented in generated code, these two features need to be baked into > the generator. This makes them special. > > You need less than that: a predicate "does have " for > certain features, ideally without baking them into the generator. > > The command registry already tracks each command's special features for > use by the policy engine. Obvious idea: also track the features you > want to pass to the predicate. > > Your series adds tracking for exactly the features you need: > > * Enumerate them in the schema with new pragma command-features > > Missing: documentation for the pragma. > > * Generate an extension QapiSpecialFeatureCustom of existing enum > QapiSpecialFeature, which is not generated. The latter is in > qapi/util.h, the former in ${prefix}qapi-init-commands.h. > > * Mark these features special for commands only, so existing registry > machinery tracks them. Do *not* make them special elsewhere, because > that would break things. > > Feels like a hack. Minor trap: if you use the same feature in > multiple schemas, multiple generated headers will define the same enum > constant, possibly with different values. If you manage to include > the wrong header *and* the value differs there, you'll likely lose > hair. > > * Missing: tests. > > I think we can avoid supplying most of the missing bits. The main QAPI > schema uses five features: deprecated, unstable, > allow-write-only-overlay, dynamic-auto-read-only, fdset. The QGA QAPI > schema uses four, namely the four you add in this series. Why not track > all features, and dispense with the pragma? Like this: > > * Change type of feature bitsets to uint64_t (it's unsigned now). > > * Error out if a schema has more than 64 features. > > * Move enum QapiSpecialFeature into a new generated header. > > * Generate a member for each feature, not just the two predefined ones. > > * Pass all command features to the registry, not just the special ones. > > * Recommended: do the same elsewhere, i.e. replace > gen_special_features() by gen_features(). > > Thoughts? So basically the code would always have access to all features, and we would have no notion of "special" features any more. I'm happy to give that a try. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|