Re: [PATCH] PoC: Rust binding for QAPI (qemu-ga only, for now)

["Marc-André Lureau" <marcandre.lureau@redhat.com>]

Hi

On Mon, Sep 21, 2020 at 1:16 PM Markus Armbruster <armbru@redhat.com> wrote:
>
> marcandre.lureau@redhat.com writes:
>
> > From: Marc-André Lureau <marcandre.lureau@redhat.com>
> >
> > 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<P>
> >   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?
>

It's not just about the D-Bus interface though.

QMP being JSON, being lazily typed: everytime we make such changes, we
inflict some pain to all the QMP bindings that want to have a
statically checked & native version of the interface. Iow, we should
think twice before doing any of this.

> > 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-André Lureau <marcandre.lureau@redhat.com>
>