Rust binding for QAPI (qemu-ga only, for now)
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 add new commands in Rust, and start
rewriting QGA in Rust, as it is a simple service. See qga/qmp/ for some
examples. QEMU would be the next obvious target.
My biggest pain-point right now is the handling of 'if' conditions. I tried
different approaches, but none of them are satisfying. I am planning to tackle
this next again, along with full QEMU API schema support and hopefully some test
integration.
v2:
- split the original patch in smaller patches and more digestable form
- dropped the DBus interface experiment from this series
- various build-sys improvements, new configure options --with-rust(-target)
- various attempts at better meson integration, finally satisfied enough with the
current solution, which handles getting link flags from Rust sanely.
(more meson stuff to come to handle config-host/features mapping etc).
- rebased, QGA QMP now uses unions, added support for it.
- start a common crate (which re-surfaced issues with foreign types and traits,
worked around with a NewPtr wrapper)
- explicit errors when ifcond are presents (after various unsucessful attempts,
I will try to tackle it in v3)
- mingw cross compilation support
- some attempts to add it to CI
- actually implement {get,set}-vcpus
- vendor the external crates