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Mon, 21 Sep 2020 09:16:12 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id BBD5B1132E9A; Mon, 21 Sep 2020 11:16:10 +0200 (CEST) From: Markus Armbruster To: marcandre.lureau@redhat.com Subject: Re: [PATCH] PoC: Rust binding for QAPI (qemu-ga only, for now) References: <20200910174850.716104-1-marcandre.lureau@redhat.com> Date: Mon, 21 Sep 2020 11:16:10 +0200 In-Reply-To: <20200910174850.716104-1-marcandre.lureau@redhat.com> (marcandre lureau's message of "Thu, 10 Sep 2020 21:48:50 +0400") Message-ID: <87mu1j8p7p.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 2.84 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=205.139.110.120; envelope-from=armbru@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/09/21 04:38:52 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=-1.501, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, 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: berrange@redhat.com, slp@redhat.com, stefanha@gmail.com, qemu-devel@nongnu.org, armbru@redhat.com, pbonzini@redhat.com, jsnow@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" marcandre.lureau@redhat.com writes: > From: Marc-Andr=C3=A9 Lureau > > Hi, > > Among the QEMU developers, there is a desire to use Rust. (see previous > 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 provide > 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? > Finally, given that the QAPI types are easy to serialize, it was simple > to use "serde" on them, and provide a D-Bus interface for QMP with zbus. > (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 * Adding results * Adding values to an enumeration type, branches to a union or alternate * Reordering members of enumerations, structs, unions * Turning an argument type into an alternate with the old type as branch 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? > This PoC modifies qemu-ga to provide the interface on the session bus: > $ qga/qemu-ga -m unix-listen -p /tmp/qga.sock -t /tmp -v > $ busctl --user introspect org.qemu.qga /org/qemu/qga org.qemu.QgaQapi > ... > $ busctl --user call org.qemu.qga /org/qemu/qga org.qemu.QgaQapi > GuestSetVcpus aa\{sv\} 1 2 logical-id x 0 online b 1 > ... > > Note: the generated code doesn't work with the qemu schema, there is a > couple of fixme/todo left. > > Shameful pain point: meson & cargo don't play nicely together. > > Signed-off-by: Marc-Andr=C3=A9 Lureau