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=-3.5 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED autolearn=no 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 DCEC1C4363D for ; Tue, 22 Sep 2020 16:11:29 +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 36EFE22206 for ; Tue, 22 Sep 2020 16:11:29 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="d3k/OaJ/" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 36EFE22206 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:35720 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1kKkt5-0002BH-TB for qemu-devel@archiver.kernel.org; Tue, 22 Sep 2020 12:11:27 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:60432) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1kKjvy-0005XW-Tg for qemu-devel@nongnu.org; Tue, 22 Sep 2020 11:10:22 -0400 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:52319) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1kKjvw-00056p-45 for qemu-devel@nongnu.org; Tue, 22 Sep 2020 11:10:22 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1600787418; 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=pr0dmzt+1CiBcbdrMHRQumCQ+FbBYktWWUxt2VotUVA=; b=d3k/OaJ/RrDMI/bJkdiqmj+qgjS35sgYKaNLaiRII7WAZwCnxWGWxMU61mPfxjEBbpezYK fjUxk+F7NjnB+GTNJTQgpx+SjR/9Sps2LCQsOF67p0CntjKNFaXm5CYnYXcMY1eFsNhPJj dvGPxeThqAUbXPTotoF8owLTCK9VMcQ= 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-364-gUu_9xrOPr2DmP9HvXYL1w-1; Tue, 22 Sep 2020 11:10:13 -0400 X-MC-Unique: gUu_9xrOPr2DmP9HvXYL1w-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 869B1801AE3; Tue, 22 Sep 2020 15:10:12 +0000 (UTC) Received: from blackfin.pond.sub.org (ovpn-114-66.ams2.redhat.com [10.36.114.66]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 1CF4A78805; Tue, 22 Sep 2020 15:09:58 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id A61251132E9A; Tue, 22 Sep 2020 17:09:56 +0200 (CEST) From: Markus Armbruster To: =?utf-8?Q?Marc-Andr=C3=A9?= Lureau Subject: Re: [PATCH] PoC: Rust binding for QAPI (qemu-ga only, for now) References: <20200910174850.716104-1-marcandre.lureau@redhat.com> <87mu1j8p7p.fsf@dusky.pond.sub.org> Date: Tue, 22 Sep 2020 17:09:56 +0200 In-Reply-To: (=?utf-8?Q?=22Marc-Andr=C3=A9?= Lureau"'s message of "Mon, 21 Sep 2020 14:04:31 +0400") Message-ID: <874knpluez.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 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=63.128.21.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/09/22 02:07:04 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -35 X-Spam_score: -3.6 X-Spam_bar: --- X-Spam_report: (-3.6 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.455, 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_H5=0.001, RCVD_IN_MSPIKE_WL=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: "P. Berrange, Daniel" , Sergio Lopez Pascual , "Hajnoczi, Stefan" , Markus Armbruster , qemu-devel , "Bonzini, Paolo" , John Snow Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Marc-Andr=C3=A9 Lureau writes: > Hi > > On Mon, Sep 21, 2020 at 1:16 PM Markus Armbruster wro= te: >> >> marcandre.lureau@redhat.com writes: >> >> > From: Marc-Andr=C3=A9 Lureau >> > >> > Hi, >> > >> > Among the QEMU developers, there is a desire to use Rust. (see previou= s >> > thread from Stefan "Why QEMU should move from C to Rust", the rust-vmm >> > related projects and other experiments). >> > >> > Thanks to our QAPI type system and the associate code generator, it is >> > relatively straightforward to create Rust bindings for the generated C >> > types (also called sys/ffi binding) and functions. (rust-bindgen could >> > probably do a similar job, but it would probably bring other issues). >> > This provides an important internal API already. >> > >> > Slightly more complicated is to expose a Rust API for those, and provi= de >> > convenient conversions C<->Rust. Taking inspiration from glib-rs >> > binding, I implemented a simplified version of the FromGlib/ToGlib >> > traits, with simpler ownership model, sufficient for QAPI needs. >> > >> > The usage is relatively simple: >> > >> > - from_qemu_none(ptr: *const sys::P) -> T >> > Return a Rust type T for a const ffi pointer P. >> > >> > - from_qemu_full(ptr: *mut sys::P) -> T >> > Return a Rust type T for a ffi pointer P, taking ownership. >> > >> > - T::to_qemu_none() -> Stash

>> > Returns a borrowed ffi pointer P (using a Stash to destroy "glue" >> > storage data, if any). >> > >> > - T::to_qemu_full() -> P >> > Returns a ffi pointer P. (P resources are leaked/passed to C/ffi) >> > >> > With those traits, it's relatively easy to implement the QMP callbacks= . >> > With enough interest, we could eventually start rewriting QGA in >> > Rust, as it is a simple service. See qga/qmp.rs for some examples. >> > We could also try to tackle qemu itself. >> >> Up to here, you're talking about *internal* interfaces. Correct? >> >> Your motivation is enabling use of Rust in QEMU. Correct? > > That's the first motivation, indeed. Sounds useful. >> > Finally, given that the QAPI types are easy to serialize, it was simpl= e >> > to use "serde" on them, and provide a D-Bus interface for QMP with zbu= s. >> > (a similar approach could probably be taken for other protocols, that >> > could be dynamically loaded... anyone like protobuf better?) >> >> QMP is an *external* interface. >> >> It supports compatible evolution: we can make certain kinds of changes >> without affecting clients. These include: >> >> * Adding optional arguments > > This would change the signature of the function, and would need an > interface version bump. > > Alternative: pass optional arguments as an extra dictionary. This is a > common idiom in D-Bus (the "a{sv}" type that maps strings to generic > values) > > Potentially, use gvariant serialization format, which has maybe type. > But gvariant isn't implemented by most D-Bus libraries (that was the > plan long time ago, but it didn't happen as people lost interest). > >> * Adding results > > Also change the signature of the function. > > However, since messages have boundaries, it is easy to ignore return valu= es. I'm not sure I understand this. The compatible change I have in mind is adding members to the complex type returned by a command. >> * Adding values to an enumeration type, branches to a union or >> alternate >> > > As long as the discriminant is represented as a string, it should be fine= . > >> * Reordering members of enumerations, structs, unions > > Again, if the discriminant is a string, it should be the same as with jso= n. > > For the members, the usage of dictionaries is required in this case > (else the type signature would change). > >> * Turning an argument type into an alternate with the old type as branch > > That would also change the function signature. > > There isn't much solution I can think of, unless we have an implicit > tagged enum for every argument, which would be quite nasty. > >> >> We've made use of this extensively. See also >> docs/devel/qapi-code-gen.txt section "Compatibility considerations." >> >> How do such changes affect clients of the proposed D-Bus interface? > > The introspection XML will always reflect the expected signature. You > should bump your interface version whenever you make incompatible > changes. How do "interface versions" work? Client's and server's version need to match, or else no go? > If this happens too often, we could also introduce a D-Bus override > mechanism to do manual translations from external interface to > internal. Greek to me :)