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helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -24 X-Spam_score: -2.5 X-Spam_bar: -- X-Spam_report: (-2.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.424, 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, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=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: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Inspired by the same-named type in Linux. This type provides the compiler with a correct view of what goes on with FFI types. In addition, it separates the glue code from the bindgen-generated code, allowing traits such as Send, Sync or Zeroable to be specified independently for C and Rust structs. Signed-off-by: Paolo Bonzini --- docs/devel/rust.rst | 34 +++++-- rust/qemu-api/src/cell.rs | 191 ++++++++++++++++++++++++++++++++++++-- 2 files changed, 210 insertions(+), 15 deletions(-) diff --git a/docs/devel/rust.rst b/docs/devel/rust.rst index e3f9e16aacb..9a621648e72 100644 --- a/docs/devel/rust.rst +++ b/docs/devel/rust.rst @@ -295,15 +295,33 @@ 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``. +to a Rust mutable reference, and use a shared reference instead. The +``qemu_api::cell`` module provides wrappers that can be used to tell the +Rust compiler about interior mutability, and optionally to enforce locking +rules for the "Big QEMU Lock". In the future, similar cell types might +also be provided for ``AioContext``-based locking as well. -In the future, similar cell types might also be provided for ``AioContext``-based -locking as well. +In particular, device code will usually rely on the ``BqlRefCell`` and +``BqlCell`` type to ensure that data is accessed correctly under the +"Big QEMU Lock". 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``. + +Bindings code instead will usually use the ``Opaque`` type, which hides +the contents of the underlying struct and can be easily converted to +a raw pointer, for use in calls to C functions. It can be used for +example as follows:: + + #[repr(transparent)] + #[derive(Debug)] + pub struct Object(Opaque); + +The bindings will then manually check for the big QEMU lock with +assertions, which allows the wrapper to be declared thread-safe:: + + unsafe impl Send for Object {} + unsafe impl Sync for Object {} Writing bindings to C code '''''''''''''''''''''''''' diff --git a/rust/qemu-api/src/cell.rs b/rust/qemu-api/src/cell.rs index eae4e2ce786..84b9eb07467 100644 --- a/rust/qemu-api/src/cell.rs +++ b/rust/qemu-api/src/cell.rs @@ -27,7 +27,7 @@ // IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER // DEALINGS IN THE SOFTWARE. -//! BQL-protected mutable containers. +//! QEMU-specific mutable containers //! //! Rust memory safety is based on this rule: Given an object `T`, it is only //! possible to have one of the following: @@ -43,8 +43,10 @@ //! usually have their pointer shared with the "outside world very early in //! their lifetime", for example when they create their //! [`MemoryRegion`s](crate::bindings::MemoryRegion). Therefore, individual -//! parts of a device must be made mutable in a controlled manner through the -//! use of cell types. +//! parts of a device must be made mutable in a controlled manner; this module +//! provides the tools to do so. +//! +//! ## Cell types //! //! [`BqlCell`] and [`BqlRefCell`] allow doing this via the Big QEMU Lock. //! While they are essentially the same single-threaded primitives that are @@ -71,7 +73,7 @@ //! QEMU device implementations is usually incorrect and can lead to //! thread-safety issues. //! -//! ## `BqlCell` +//! ### `BqlCell` //! //! [`BqlCell`] implements interior mutability by moving values in and out of //! the cell. That is, an `&mut T` to the inner value can never be obtained as @@ -91,7 +93,7 @@ //! - [`set`](BqlCell::set): this method replaces the interior value, //! dropping the replaced value. //! -//! ## `BqlRefCell` +//! ### `BqlRefCell` //! //! [`BqlRefCell`] uses Rust's lifetimes to implement "dynamic borrowing", a //! process whereby one can claim temporary, exclusive, mutable access to the @@ -111,13 +113,82 @@ //! Multiple immutable borrows are allowed via [`borrow`](BqlRefCell::borrow), //! or a single mutable borrow via [`borrow_mut`](BqlRefCell::borrow_mut). The //! thread will panic if these rules are violated or if the BQL is not held. +//! +//! ## Opaque wrappers +//! +//! The cell types from the previous section are useful at the boundaries +//! of code that requires interior mutability. When writing glue code that +//! interacts directly with C structs, however, it is useful to operate +//! at a lower level. +//! +//! C functions often violate Rust's fundamental assumptions about memory +//! safety by modifying memory even if it is shared. Furthermore, C structs +//! often start their life uninitialized and may be populated lazily. +//! +//! For this reason, this module provides the [`Opaque`] type to opt out +//! of Rust's usual guarantees about the wrapped type. Access to the wrapped +//! value is always through raw pointers, obtained via methods like +//! [`as_mut_ptr()`](Opaque::as_mut_ptr) and [`as_ptr()`](Opaque::as_ptr). These +//! pointers can then be passed to C functions or dereferenced; both actions +//! require `unsafe` blocks, making it clear where safety guarantees must be +//! manually verified. For example +//! +//! ```ignore +//! let state = Opaque::::uninit(); +//! unsafe { +//! qemu_struct_init(state.as_mut_ptr()); +//! } +//! ``` +//! +//! [`Opaque`] will usually be wrapped one level further, so that +//! bridge methods can be added to the wrapper: +//! +//! ```ignore +//! pub struct MyStruct(Opaque); +//! +//! impl MyStruct { +//! fn new() -> Pin> { +//! let result = Box::pin(Opaque::uninit()); +//! unsafe { qemu_struct_init(result.as_mut_ptr()) }; +//! result +//! } +//! } +//! ``` +//! +//! This pattern of wrapping bindgen-generated types in [`Opaque`] provides +//! several advantages: +//! +//! * The choice of traits to be implemented is not limited by the +//! bindgen-generated code. For example, [`Drop`] can be added without +//! disabling [`Copy`] on the underlying bindgen type +//! +//! * [`Send`] and [`Sync`] implementations can be controlled by the wrapper +//! type rather than being automatically derived from the C struct's layout +//! +//! * Methods can be implemented in a separate crate from the bindgen-generated +//! bindings +//! +//! * [`Debug`](std::fmt::Debug) and [`Display`](std::fmt::Display) +//! implementations can be customized to be more readable than the raw C +//! struct representation +//! +//! The [`Opaque`] type does not include BQL validation; it is possible to +//! assert in the code that the right lock is taken, to use it together +//! with a custom lock guard type, or to let C code take the lock, as +//! appropriate. It is also possible to use it with non-thread-safe +//! types, since by default (unlike [`BqlCell`] and [`BqlRefCell`] +//! it is neither `Sync` nor `Send`. +//! +//! While [`Opaque`] is necessary for C interop, it should be used sparingly +//! and only at FFI boundaries. For QEMU-specific types that need interior +//! mutability, prefer [`BqlCell`] or [`BqlRefCell`]. use std::{ cell::{Cell, UnsafeCell}, cmp::Ordering, fmt, - marker::PhantomData, - mem, + marker::{PhantomData, PhantomPinned}, + mem::{self, MaybeUninit}, ops::{Deref, DerefMut}, ptr::NonNull, }; @@ -840,3 +911,109 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { (**self).fmt(f) } } + +/// Stores an opaque value that is shared with C code. +/// +/// Often, C structs can changed when calling a C function even if they are +/// behind a shared Rust reference, or they can be initialized lazily and have +/// invalid bit patterns (e.g. `3` for a [`bool`]). This goes against Rust's +/// strict aliasing rules, which normally prevent mutation through shared +/// references. +/// +/// Wrapping the struct with `Opaque` ensures that the Rust compiler does not +/// assume the usual constraints that Rust structs require, and allows using +/// shared references on the Rust side. +/// +/// `Opaque` is `#[repr(transparent)]`, so that it matches the memory layout +/// of `T`. +#[repr(transparent)] +pub struct Opaque { + value: UnsafeCell>, + // PhantomPinned also allows multiple references to the `Opaque`, i.e. + // one `&mut Opaque` can coexist with a `&mut T` or any number of `&T`; + // see https://docs.rs/pinned-aliasable/latest/pinned_aliasable/. + _pin: PhantomPinned, +} + +impl Opaque { + /// Creates a new shared reference from a C pointer + /// + /// # Safety + /// + /// The pointer must be valid, though it need not point to a valid value. + pub unsafe fn from_raw<'a>(ptr: *mut T) -> &'a Self { + let ptr = NonNull::new(ptr).unwrap().cast::(); + // SAFETY: Self is a transparent wrapper over T + unsafe { ptr.as_ref() } + } + + /// Creates a new opaque object with uninitialized contents. + /// + /// # Safety + /// + /// Ultimately the pointer to the returned value will be dereferenced + /// in another unsafe block, for example when passing it to a C function. + /// However, this function is unsafe to "force" documenting who is going + /// to initialize and pin the value. + pub const unsafe fn uninit() -> Self { + Self { + value: UnsafeCell::new(MaybeUninit::uninit()), + _pin: PhantomPinned, + } + } + + /// Creates a new opaque object with zeroed contents. + /// + /// # Safety + /// + /// Ultimately the pointer to the returned value will be dereferenced + /// in another unsafe block, for example when passing it to a C function. + /// However, this function is unsafe to "force" documenting whether a + /// zero value is safe. + pub const unsafe fn zeroed() -> Self { + Self { + value: UnsafeCell::new(MaybeUninit::uninit()), + _pin: PhantomPinned, + } + } + + /// Returns a raw pointer to the opaque data. + pub const fn as_mut_ptr(&self) -> *mut T { + UnsafeCell::get(&self.value).cast() + } + + /// Returns a raw pointer to the opaque data. + pub const fn as_ptr(&self) -> *const T { + self.as_mut_ptr() as *const _ + } + + /// Returns a raw pointer to the opaque data. + pub const fn as_void_ptr(&self) -> *mut std::ffi::c_void { + UnsafeCell::get(&self.value).cast() + } +} + +impl fmt::Debug for Opaque { + fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { + let mut name: String = "Opaque<".to_string(); + name += std::any::type_name::(); + name += ">"; + f.debug_tuple(&name).field(&self.as_ptr()).finish() + } +} + +impl Default for Opaque { + fn default() -> Self { + Self { + value: UnsafeCell::new(MaybeUninit::new(T::default())), + _pin: PhantomPinned, + } + } +} + +impl Opaque { + /// Creates a new opaque object with default contents. + pub fn new() -> Self { + Self::default() + } +}