Rust in QEMU

Rust in QEMU is a project to enable using the Rust programming language to add new functionality to QEMU.

Right now, the focus is on making it possible to write devices that inherit from SysBusDevice in *safe* Rust. Later, it may become possible to write other kinds of devices (e.g. PCI devices that can do DMA), complete boards, or backends (e.g. block device formats).

Building the Rust in QEMU code

The Rust in QEMU code is included in the emulators via Meson. Meson invokes rustc directly, building static libraries that are then linked together with the C code. This is completely automatic when you run make or ninja.

However, QEMU’s build system also tries to be easy to use for people who are accustomed to the more “normal” Cargo-based development workflow. In particular:

the set of warnings and lints that are used to build QEMU always comes from the rust/Cargo.toml workspace file
it is also possible to use cargo for common Rust-specific coding tasks, in particular to invoke clippy, rustfmt and rustdoc.

To this end, QEMU includes a build.rs build script that picks up generated sources from QEMU’s build directory and puts it in Cargo’s output directory (typically rust/target/). A vanilla invocation of Cargo will complain that it cannot find the generated sources, which can be fixed in different ways:

by using special shorthand targets in the QEMU build directory:
```
make clippy
make rustfmt
make rustdoc
```
by invoking cargo through the Meson development environment feature:
```
pyvenv/bin/meson devenv -w ../rust cargo clippy --tests
pyvenv/bin/meson devenv -w ../rust cargo fmt
```
If you are going to use cargo repeatedly, pyvenv/bin/meson devenv will enter a shell where commands like cargo clippy just work.

by pointing the MESON_BUILD_ROOT to the top of your QEMU build tree. This third method is useful if you are using rust-analyzer; you can set the environment variable through the rust-analyzer.cargo.extraEnv setting.

As shown above, you can use the --tests option as usual to operate on test code. Note however that you cannot build or run tests via cargo, because they need support C code from QEMU that Cargo does not know about. Tests can be run via meson test or make:

make check-rust

Building Rust code with --enable-modules is not supported yet.

Supported tools

QEMU supports rustc version 1.63.0 and newer. Notably, the following features are missing:

core::ffi (1.64.0). Use std::os::raw and std::ffi instead.
cast_mut()/cast_const() (1.65.0). Use as instead.
“let … else” (1.65.0). Use if let instead. This is currently patched in QEMU’s vendored copy of the bilge crate.
Generic Associated Types (1.65.0)
CStr::from_bytes_with_nul() as a const function (1.72.0).
“Return position impl Trait in Traits” (1.75.0, blocker for including the pinned-init create).
MaybeUninit::zeroed() as a const function (1.75.0). QEMU’s Zeroable trait can be implemented without MaybeUninit::zeroed(), so this would be just a cleanup.
c"" literals (stable in 1.77.0). QEMU provides a c_str!() macro to define CStr constants easily
offset_of! (stable in 1.77.0). QEMU uses offset_of!() heavily; it provides a replacement in the qemu_api crate, but it does not support lifetime parameters and therefore &'a Something fields in the struct may have to be replaced by NonNull<Something>. Nested offset_of! was only stabilized in Rust 1.82.0, but it is not used.
inline const expression (stable in 1.79.0), currently worked around with associated constants in the FnCall trait.
associated constants have to be explicitly marked 'static (changed in 1.81.0)
&raw (stable in 1.82.0). Use addr_of! and addr_of_mut! instead, though hopefully the need for raw pointers will go down over time.
new_uninit (stable in 1.82.0). This is used internally by the pinned_init crate, which is planned for inclusion in QEMU, but it can be easily patched out.
referencing statics in constants (stable in 1.83.0). For now use a const function; this is an important limitation for QEMU’s migration stream architecture (VMState). Right now, VMState lacks type safety because it is hard to place the VMStateField definitions in traits.
associated const equality would be nice to have for some users of callbacks::FnCall, but is still experimental. ASSERT_IS_SOME replaces it.

It is expected that QEMU will advance its minimum supported version of rustc to 1.77.0 as soon as possible; as of January 2025, blockers for that right now are Debian bookworm and 32-bit MIPS processors. This unfortunately means that references to statics in constants will remain an issue.

QEMU also supports version 0.60.x of bindgen, which is missing option --generate-cstr. This option requires version 0.66.x and will be adopted as soon as supporting these older versions is not necessary anymore.

Writing Rust code in QEMU

Right now QEMU includes three crates:

qemu_api for bindings to C code and useful functionality
qemu_api_macros defines several procedural macros that are useful when writing C code
pl011 (under rust/hw/char/pl011) is the sample device that is being used to further develop qemu_api and qemu_api_macros. It is a functional replacement for the hw/char/pl011.c file.

This section explains how to work with them.

Status

Modules of qemu_api can be defined as:

complete: ready for use in new devices; if applicable, the API supports the full functionality available in C
stable: ready for production use, the API is safe and should not undergo major changes
proof of concept: the API is subject to change but allows working with safe Rust
initial: the API is in its initial stages; it requires large amount of unsafe code; it might have soundness or type-safety issues

The status of the modules is as follows:

module	status
`assertions`	stable
`bitops`	complete
`callbacks`	complete
`cell`	stable
`c_str`	complete
`irq`	complete
`memory`	stable
`module`	complete
`offset_of`	stable
`qdev`	stable
`qom`	stable
`sysbus`	stable
`timer`	stable
`vmstate`	proof of concept
`zeroable`	stable

Note

API stability is not a promise, if anything because the C APIs are not a stable interface either. Also, unsafe interfaces may be replaced by safe interfaces later.

Naming convention

C function names usually are prefixed according to the data type that they apply to, for example timer_mod or sysbus_connect_irq. Furthermore, both function and structs sometimes have a qemu_ or QEMU prefix. Generally speaking, these are all removed in the corresponding Rust functions: QEMUTimer becomes timer::Timer, timer_mod becomes Timer::modify, sysbus_connect_irq becomes SysBusDeviceMethods::connect_irq.

Sometimes however a name appears multiple times in the QOM class hierarchy, and the only difference is in the prefix. An example is qdev_realize and sysbus_realize. In such cases, whenever a name is not unique in the hierarchy, always add the prefix to the classes that are lower in the hierarchy; for the top class, decide on a case by case basis.

For example:

`device_cold_reset()`	`DeviceMethods::cold_reset()`
`pci_device_reset()`	`PciDeviceMethods::pci_device_reset()`
`pci_bridge_reset()`	`PciBridgeMethods::pci_bridge_reset()`

Here, the name is not exactly the same, but nevertheless PciDeviceMethods adds the prefix to avoid confusion, because the functionality of device_cold_reset() and pci_device_reset() is subtly different.

In this case, however, no prefix is needed:

`device_realize()`	`DeviceMethods::realize()`
`sysbus_realize()`	`SysbusDeviceMethods::sysbus_realize()`
`pci_realize()`	`PciDeviceMethods::pci_realize()`

Here, the lower classes do not add any functionality, and mostly provide extra compile-time checking; the basic realize functionality is the same for all devices. Therefore, DeviceMethods does not add the prefix.

Whenever a name is unique in the hierarchy, instead, you should always remove the class name prefix.

Common pitfalls

Rust has very strict rules with respect to how you get an exclusive (&mut) reference; failure to respect those rules is a source of undefined behavior. In particular, even if a value is loaded from a raw mutable pointer (*mut), it cannot be casted to &mut unless the value was stored to the *mut from a mutable reference. Furthermore, it is undefined behavior if any shared reference was created between the store to the *mut and the load:

let mut p: u32 = 42;
let p_mut = &mut p;                              // 1
let p_raw = p_mut as *mut u32;                   // 2

// p_raw keeps the mutable reference "alive"

let p_shared = &p;                               // 3
println!("access from &u32: {}", *p_shared);

// Bring back the mutable reference, its lifetime overlaps
// with that of a shared reference.
let p_mut = unsafe { &mut *p_raw };              // 4
println!("access from &mut 32: {}", *p_mut);

println!("access from &u32: {}", *p_shared);     // 5

These rules can be tested with MIRI, for example.

Almost all Rust code in QEMU will involve QOM objects, and pointers to these objects are shared, for example because they are part of the QOM composition tree. This creates exactly the above scenario:

a QOM object is created
a *mut is created, for example as the opaque value for a MemoryRegion
the QOM object is placed in the composition tree
a memory access dereferences the opaque value to a &mut
but the shared reference is still present in the composition tree

Because of this, QOM objects should almost always use &self instead of &mut self; access to internal fields must use interior mutability to go from a shared reference to a &mut.

Whenever C code provides you with an opaque void *, avoid converting it to a Rust mutable reference, and use a shared reference instead. Rust code will then have to use QEMU’s BqlRefCell and BqlCell type, which enforce that locking rules for the “Big QEMU Lock” are respected. These cell types are also known to the vmstate crate, which is able to “look inside” them when building an in-memory representation of a struct``s layout. Note that the same is not true of a ``RefCell or Mutex.

In the future, similar cell types might also be provided for AioContext-based locking as well.

Writing bindings to C code

Here are some things to keep in mind when working on the qemu_api crate.

Look at existing code: Very often, similar idioms in C code correspond to similar tricks in Rust bindings. If the C code uses offsetof, look at qdev properties or vmstate. If the C code has a complex const struct, look at MemoryRegion. Reuse existing patterns for handling lifetimes; for example use &T for QOM objects that do not need a reference count (including those that can be embedded in other objects) and Owned<T> for those that need it.
Use the type system: Bindings often will need access information that is specific to a type (either a builtin one or a user-defined one) in order to pass it to C functions. Put them in a trait and access it through generic parameters. The vmstate module has examples of how to retrieve type information for the fields of a Rust struct.
Prefer unsafe traits to unsafe functions: Unsafe traits are much easier to prove correct than unsafe functions. They are an excellent place to store metadata that can later be accessed by generic functions. C code usually places metadata in global variables; in Rust, they can be stored in traits and then turned into static variables. Often, unsafe traits can be generated by procedural macros.
Document limitations due to old Rust versions: If you need to settle for an inferior solution because of the currently supported set of Rust versions, document it in the source and in this file. This ensures that it can be fixed when the minimum supported version is bumped.
Keep locking in mind.: When marking a type Sync, be careful of whether it needs the big QEMU lock. Use BqlCell and BqlRefCell for interior data, or assert bql_locked().
Don’t be afraid of complexity, but document and isolate it: It’s okay to be tricky; device code is written more often than bindings code and it’s important that it is idiomatic. However, you should strive to isolate any tricks in a place (for example a struct, a trait or a macro) where it can be documented and tested. If needed, include toy versions of the code in the documentation.

Writing procedural macros

By conventions, procedural macros are split in two functions, one returning Result<proc_macro2::TokenStream, MacroError>` with the body of the procedural macro, and the second returning ``proc_macro::TokenStream which is the actual procedural macro. The former’s name is the same as the latter with the _or_error suffix. The code for the latter is more or less fixed; it follows the following template, which is fixed apart from the type after as in the invocation of parse_macro_input!:

#[proc_macro_derive(Object)]
pub fn derive_object(input: TokenStream) -> TokenStream {
    let input = parse_macro_input!(input as DeriveInput);
    let expanded = derive_object_or_error(input).unwrap_or_else(Into::into);

    TokenStream::from(expanded)
}

The qemu_api_macros crate has utility functions to examine a DeriveInput and perform common checks (e.g. looking for a struct with named fields). These functions return Result<..., MacroError> and can be used easily in the procedural macro function:

fn derive_object_or_error(input: DeriveInput) ->
    Result<proc_macro2::TokenStream, MacroError>
{
    is_c_repr(&input, "#[derive(Object)]")?;

    let name = &input.ident;
    let parent = &get_fields(&input, "#[derive(Object)]")?[0].ident;
    ...
}

Use procedural macros with care. They are mostly useful for two purposes:

Performing consistency checks; for example #[derive(Object)] checks that the structure has #[repr[C]) and that the type of the first field is consistent with the ObjectType declaration.
Extracting information from Rust source code into traits, typically based on types and attributes. For example, #[derive(TryInto)] builds an implementation of TryFrom, and it uses the #[repr(...)] attribute as the TryFrom source and error types.

Procedural macros can be hard to debug and test; if the code generation exceeds a few lines of code, it may be worthwhile to delegate work to “regular” declarative (macro_rules!) macros and write unit tests for those instead.

Coding style

Code should pass clippy and be formatted with rustfmt.

Right now, only the nightly version of rustfmt is supported. This might change in the future. While CI checks for correct formatting via cargo fmt --check, maintainers can fix this for you when applying patches.

It is expected that qemu_api provides full rustdoc documentation for bindings that are in their final shape or close.

Adding dependencies

Generally, the set of dependent crates is kept small. Think twice before adding a new external crate, especially if it comes with a large set of dependencies itself. Sometimes QEMU only needs a small subset of the functionality; see for example QEMU’s assertions or c_str modules.

On top of this recommendation, adding external crates to QEMU is a slightly complicated process, mostly due to the need to teach Meson how to build them. While Meson has initial support for parsing Cargo.lock files, it is still highly experimental and is therefore not used.

Therefore, external crates must be added as subprojects for Meson to learn how to build them, as well as to the relevant Cargo.toml files. The versions specified in rust/Cargo.lock must be the same as the subprojects; note that the rust/ directory forms a Cargo workspace, and therefore there is a single lock file for the whole build.

Choose a version of the crate that works with QEMU’s minimum supported Rust version (1.63.0).

Second, a new wrap file must be added to teach Meson how to download the crate. The wrap file must be named NAME-SEMVER-rs.wrap, where NAME is the name of the crate and SEMVER is the version up to and including the first non-zero number. For example, a crate with version 0.2.3 will use 0.2 for its SEMVER, while a crate with version 1.0.84 will use 1.

Third, the Meson rules to build the crate must be added at subprojects/NAME-SEMVER-rs/meson.build. Generally this includes:

subproject and dependency lines for all dependent crates
a static_library or rust.proc_macro line to perform the actual build
declare_dependency and a meson.override_dependency lines to expose the result to QEMU and to other subprojects

Remember to add native: true to dependency, static_library and meson.override_dependency for dependencies of procedural macros. If a crate is needed in both procedural macros and QEMU binaries, everything apart from subproject must be duplicated to build both native and non-native versions of the crate.

It’s important to specify the right compiler options. These include:

the language edition (which can be found in the Cargo.toml file)
the --cfg (which have to be “reverse engineered” from the build.rs file of the crate).
usually, a --cap-lints allow argument to hide warnings from rustc or clippy.

After every change to the meson.build file you have to update the patched version with meson subprojects update --reset ``NAME-SEMVER-rs. This might be automated in the future.

Also, after every change to the meson.build file it is strongly suggested to do a dummy change to the .wrap file (for example adding a comment like # version 2), which will help Meson notice that the subproject is out of date.

As a last step, add the new subproject to scripts/archive-source.sh, scripts/make-release and subprojects/.gitignore.