//! parts of a device must be made mutable in a controlled manner through the
//! use of cell types.
//!
-//! This module provides a way to do so via the Big QEMU Lock. While
-//! [`BqlCell<T>`] is essentially the same single-threaded primitive that is
-//! available in `std::cell`, the BQL allows it to be used from a multi-threaded
-//! context and to share references across threads, while maintaining Rust's
-//! safety guarantees. For this reason, unlike its `std::cell` counterpart,
-//! `BqlCell` implements the `Sync` trait.
+//! [`BqlCell<T>`] and [`BqlRefCell<T>`] allow doing this via the Big QEMU Lock.
+//! While they are essentially the same single-threaded primitives that are
+//! available in `std::cell`, the BQL allows them to be used from a
+//! multi-threaded context and to share references across threads, while
+//! maintaining Rust's safety guarantees. For this reason, unlike
+//! their `std::cell` counterparts, `BqlCell` and `BqlRefCell` implement the
+//! `Sync` trait.
//!
//! BQL checks are performed in debug builds but can be optimized away in
//! release builds, providing runtime safety during development with no overhead
//! in production.
//!
-//! Warning: While `BqlCell` is similar to its `std::cell` counterpart, the two
-//! are not interchangeable. Using `std::cell` types in QEMU device
-//! implementations is usually incorrect and can lead to thread-safety issues.
+//! The two provide different ways of handling interior mutability.
+//! `BqlRefCell` is best suited for data that is primarily accessed by the
+//! device's own methods, where multiple reads and writes can be grouped within
+//! a single borrow and a mutable reference can be passed around. Instead,
+//! [`BqlCell`] is a better choice when sharing small pieces of data with
+//! external code (especially C code), because it provides simple get/set
+//! operations that can be used one at a time.
+//!
+//! Warning: While `BqlCell` and `BqlRefCell` are similar to their `std::cell`
+//! counterparts, they are not interchangeable. Using `std::cell` types in
+//! QEMU device implementations is usually incorrect and can lead to
+//! thread-safety issues.
//!
//! ## `BqlCell<T>`
//!
//! returns the replaced value.
//! - [`set`](BqlCell::set): this method replaces the interior value,
//! dropping the replaced value.
+//!
+//! ## `BqlRefCell<T>`
+//!
+//! [`BqlRefCell<T>`] uses Rust's lifetimes to implement "dynamic borrowing", a
+//! process whereby one can claim temporary, exclusive, mutable access to the
+//! inner value:
+//!
+//! ```ignore
+//! fn clear_interrupts(&self, val: u32) {
+//! // A mutable borrow gives read-write access to the registers
+//! let mut regs = self.registers.borrow_mut();
+//! let old = regs.interrupt_status();
+//! regs.update_interrupt_status(old & !val);
+//! }
+//! ```
+//!
+//! Borrows for `BqlRefCell<T>`s are tracked at _runtime_, unlike Rust's native
+//! reference types which are entirely tracked statically, at compile time.
+//! 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.
-use std::{cell::UnsafeCell, cmp::Ordering, fmt, mem};
+use std::{
+ cell::{Cell, UnsafeCell},
+ cmp::Ordering,
+ fmt,
+ marker::PhantomData,
+ mem,
+ ops::{Deref, DerefMut},
+ ptr::NonNull,
+};
use crate::bindings;
!cfg!(MESON) || unsafe { bindings::bql_locked() }
}
+fn bql_block_unlock(increase: bool) {
+ if cfg!(MESON) {
+ // SAFETY: this only adjusts a counter
+ unsafe {
+ bindings::bql_block_unlock(increase);
+ }
+ }
+}
+
/// A mutable memory location that is protected by the Big QEMU Lock.
///
/// # Memory layout
self.replace(Default::default())
}
}
+
+/// A mutable memory location with dynamically checked borrow rules,
+/// protected by the Big QEMU Lock.
+///
+/// See the [module-level documentation](self) for more.
+///
+/// # Memory layout
+///
+/// `BqlRefCell<T>` starts with the same in-memory representation as its
+/// inner type `T`.
+#[repr(C)]
+pub struct BqlRefCell<T> {
+ // It is important that this is the first field (which is not the case
+ // for std::cell::BqlRefCell), so that we can use offset_of! on it.
+ // UnsafeCell and repr(C) both prevent usage of niches.
+ value: UnsafeCell<T>,
+ borrow: Cell<BorrowFlag>,
+ // Stores the location of the earliest currently active borrow.
+ // This gets updated whenever we go from having zero borrows
+ // to having a single borrow. When a borrow occurs, this gets included
+ // in the panic message
+ #[cfg(feature = "debug_cell")]
+ borrowed_at: Cell<Option<&'static std::panic::Location<'static>>>,
+}
+
+// Positive values represent the number of `BqlRef` active. Negative values
+// represent the number of `BqlRefMut` active. Right now QEMU's implementation
+// does not allow to create `BqlRefMut`s that refer to distinct, nonoverlapping
+// components of a `BqlRefCell` (e.g., different ranges of a slice).
+//
+// `BqlRef` and `BqlRefMut` are both two words in size, and so there will likely
+// never be enough `BqlRef`s or `BqlRefMut`s in existence to overflow half of
+// the `usize` range. Thus, a `BorrowFlag` will probably never overflow or
+// underflow. However, this is not a guarantee, as a pathological program could
+// repeatedly create and then mem::forget `BqlRef`s or `BqlRefMut`s. Thus, all
+// code must explicitly check for overflow and underflow in order to avoid
+// unsafety, or at least behave correctly in the event that overflow or
+// underflow happens (e.g., see BorrowRef::new).
+type BorrowFlag = isize;
+const UNUSED: BorrowFlag = 0;
+
+#[inline(always)]
+const fn is_writing(x: BorrowFlag) -> bool {
+ x < UNUSED
+}
+
+#[inline(always)]
+const fn is_reading(x: BorrowFlag) -> bool {
+ x > UNUSED
+}
+
+impl<T> BqlRefCell<T> {
+ /// Creates a new `BqlRefCell` containing `value`.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use qemu_api::cell::BqlRefCell;
+ ///
+ /// let c = BqlRefCell::new(5);
+ /// ```
+ #[inline]
+ pub const fn new(value: T) -> BqlRefCell<T> {
+ BqlRefCell {
+ value: UnsafeCell::new(value),
+ borrow: Cell::new(UNUSED),
+ #[cfg(feature = "debug_cell")]
+ borrowed_at: Cell::new(None),
+ }
+ }
+}
+
+// This ensures the panicking code is outlined from `borrow_mut` for
+// `BqlRefCell`.
+#[inline(never)]
+#[cold]
+#[cfg(feature = "debug_cell")]
+fn panic_already_borrowed(source: &Cell<Option<&'static std::panic::Location<'static>>>) -> ! {
+ // If a borrow occurred, then we must already have an outstanding borrow,
+ // so `borrowed_at` will be `Some`
+ panic!("already borrowed at {:?}", source.take().unwrap())
+}
+
+#[inline(never)]
+#[cold]
+#[cfg(not(feature = "debug_cell"))]
+fn panic_already_borrowed() -> ! {
+ panic!("already borrowed")
+}
+
+impl<T> BqlRefCell<T> {
+ #[inline]
+ #[allow(clippy::unused_self)]
+ fn panic_already_borrowed(&self) -> ! {
+ #[cfg(feature = "debug_cell")]
+ {
+ panic_already_borrowed(&self.borrowed_at)
+ }
+ #[cfg(not(feature = "debug_cell"))]
+ {
+ panic_already_borrowed()
+ }
+ }
+
+ /// Immutably borrows the wrapped value.
+ ///
+ /// The borrow lasts until the returned `BqlRef` exits scope. Multiple
+ /// immutable borrows can be taken out at the same time.
+ ///
+ /// # Panics
+ ///
+ /// Panics if the value is currently mutably borrowed.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use qemu_api::cell::BqlRefCell;
+ ///
+ /// let c = BqlRefCell::new(5);
+ ///
+ /// let borrowed_five = c.borrow();
+ /// let borrowed_five2 = c.borrow();
+ /// ```
+ ///
+ /// An example of panic:
+ ///
+ /// ```should_panic
+ /// use qemu_api::cell::BqlRefCell;
+ ///
+ /// let c = BqlRefCell::new(5);
+ ///
+ /// let m = c.borrow_mut();
+ /// let b = c.borrow(); // this causes a panic
+ /// ```
+ #[inline]
+ #[track_caller]
+ pub fn borrow(&self) -> BqlRef<'_, T> {
+ if let Some(b) = BorrowRef::new(&self.borrow) {
+ // `borrowed_at` is always the *first* active borrow
+ if b.borrow.get() == 1 {
+ #[cfg(feature = "debug_cell")]
+ self.borrowed_at.set(Some(std::panic::Location::caller()));
+ }
+
+ bql_block_unlock(true);
+
+ // SAFETY: `BorrowRef` ensures that there is only immutable access
+ // to the value while borrowed.
+ let value = unsafe { NonNull::new_unchecked(self.value.get()) };
+ BqlRef { value, borrow: b }
+ } else {
+ self.panic_already_borrowed()
+ }
+ }
+
+ /// Mutably borrows the wrapped value.
+ ///
+ /// The borrow lasts until the returned `BqlRefMut` or all `BqlRefMut`s
+ /// derived from it exit scope. The value cannot be borrowed while this
+ /// borrow is active.
+ ///
+ /// # Panics
+ ///
+ /// Panics if the value is currently borrowed.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use qemu_api::cell::BqlRefCell;
+ ///
+ /// let c = BqlRefCell::new("hello".to_owned());
+ ///
+ /// *c.borrow_mut() = "bonjour".to_owned();
+ ///
+ /// assert_eq!(&*c.borrow(), "bonjour");
+ /// ```
+ ///
+ /// An example of panic:
+ ///
+ /// ```should_panic
+ /// use qemu_api::cell::BqlRefCell;
+ ///
+ /// let c = BqlRefCell::new(5);
+ /// let m = c.borrow();
+ ///
+ /// let b = c.borrow_mut(); // this causes a panic
+ /// ```
+ #[inline]
+ #[track_caller]
+ pub fn borrow_mut(&self) -> BqlRefMut<'_, T> {
+ if let Some(b) = BorrowRefMut::new(&self.borrow) {
+ #[cfg(feature = "debug_cell")]
+ {
+ self.borrowed_at.set(Some(std::panic::Location::caller()));
+ }
+
+ // SAFETY: this only adjusts a counter
+ bql_block_unlock(true);
+
+ // SAFETY: `BorrowRefMut` guarantees unique access.
+ let value = unsafe { NonNull::new_unchecked(self.value.get()) };
+ BqlRefMut {
+ value,
+ _borrow: b,
+ marker: PhantomData,
+ }
+ } else {
+ self.panic_already_borrowed()
+ }
+ }
+
+ /// Returns a raw pointer to the underlying data in this cell.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use qemu_api::cell::BqlRefCell;
+ ///
+ /// let c = BqlRefCell::new(5);
+ ///
+ /// let ptr = c.as_ptr();
+ /// ```
+ #[inline]
+ pub const fn as_ptr(&self) -> *mut T {
+ self.value.get()
+ }
+}
+
+// SAFETY: Same as for std::sync::Mutex. In the end this is a Mutex that is
+// stored out-of-line. Even though BqlRefCell includes Cells, they are
+// themselves protected by the Big QEMU Lock. Furtheremore, the Big QEMU
+// Lock cannot be released while any borrows is active.
+unsafe impl<T> Send for BqlRefCell<T> where T: Send {}
+unsafe impl<T> Sync for BqlRefCell<T> {}
+
+impl<T: Clone> Clone for BqlRefCell<T> {
+ /// # Panics
+ ///
+ /// Panics if the value is currently mutably borrowed.
+ #[inline]
+ #[track_caller]
+ fn clone(&self) -> BqlRefCell<T> {
+ BqlRefCell::new(self.borrow().clone())
+ }
+
+ /// # Panics
+ ///
+ /// Panics if `source` is currently mutably borrowed.
+ #[inline]
+ #[track_caller]
+ fn clone_from(&mut self, source: &Self) {
+ self.value.get_mut().clone_from(&source.borrow())
+ }
+}
+
+impl<T: Default> Default for BqlRefCell<T> {
+ /// Creates a `BqlRefCell<T>`, with the `Default` value for T.
+ #[inline]
+ fn default() -> BqlRefCell<T> {
+ BqlRefCell::new(Default::default())
+ }
+}
+
+impl<T: PartialEq> PartialEq for BqlRefCell<T> {
+ /// # Panics
+ ///
+ /// Panics if the value in either `BqlRefCell` is currently mutably
+ /// borrowed.
+ #[inline]
+ fn eq(&self, other: &BqlRefCell<T>) -> bool {
+ *self.borrow() == *other.borrow()
+ }
+}
+
+impl<T: Eq> Eq for BqlRefCell<T> {}
+
+impl<T: PartialOrd> PartialOrd for BqlRefCell<T> {
+ /// # Panics
+ ///
+ /// Panics if the value in either `BqlRefCell` is currently mutably
+ /// borrowed.
+ #[inline]
+ fn partial_cmp(&self, other: &BqlRefCell<T>) -> Option<Ordering> {
+ self.borrow().partial_cmp(&*other.borrow())
+ }
+}
+
+impl<T: Ord> Ord for BqlRefCell<T> {
+ /// # Panics
+ ///
+ /// Panics if the value in either `BqlRefCell` is currently mutably
+ /// borrowed.
+ #[inline]
+ fn cmp(&self, other: &BqlRefCell<T>) -> Ordering {
+ self.borrow().cmp(&*other.borrow())
+ }
+}
+
+impl<T> From<T> for BqlRefCell<T> {
+ /// Creates a new `BqlRefCell<T>` containing the given value.
+ fn from(t: T) -> BqlRefCell<T> {
+ BqlRefCell::new(t)
+ }
+}
+
+struct BorrowRef<'b> {
+ borrow: &'b Cell<BorrowFlag>,
+}
+
+impl<'b> BorrowRef<'b> {
+ #[inline]
+ fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
+ let b = borrow.get().wrapping_add(1);
+ if !is_reading(b) {
+ // Incrementing borrow can result in a non-reading value (<= 0) in these cases:
+ // 1. It was < 0, i.e. there are writing borrows, so we can't allow a read
+ // borrow due to Rust's reference aliasing rules
+ // 2. It was isize::MAX (the max amount of reading borrows) and it overflowed
+ // into isize::MIN (the max amount of writing borrows) so we can't allow an
+ // additional read borrow because isize can't represent so many read borrows
+ // (this can only happen if you mem::forget more than a small constant amount
+ // of `BqlRef`s, which is not good practice)
+ None
+ } else {
+ // Incrementing borrow can result in a reading value (> 0) in these cases:
+ // 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read
+ // borrow
+ // 2. It was > 0 and < isize::MAX, i.e. there were read borrows, and isize is
+ // large enough to represent having one more read borrow
+ borrow.set(b);
+ Some(BorrowRef { borrow })
+ }
+ }
+}
+
+impl Drop for BorrowRef<'_> {
+ #[inline]
+ fn drop(&mut self) {
+ let borrow = self.borrow.get();
+ debug_assert!(is_reading(borrow));
+ self.borrow.set(borrow - 1);
+ bql_block_unlock(false)
+ }
+}
+
+impl Clone for BorrowRef<'_> {
+ #[inline]
+ fn clone(&self) -> Self {
+ BorrowRef::new(self.borrow).unwrap()
+ }
+}
+
+/// Wraps a borrowed reference to a value in a `BqlRefCell` box.
+/// A wrapper type for an immutably borrowed value from a `BqlRefCell<T>`.
+///
+/// See the [module-level documentation](self) for more.
+pub struct BqlRef<'b, T: 'b> {
+ // NB: we use a pointer instead of `&'b T` to avoid `noalias` violations, because a
+ // `BqlRef` argument doesn't hold immutability for its whole scope, only until it drops.
+ // `NonNull` is also covariant over `T`, just like we would have with `&T`.
+ value: NonNull<T>,
+ borrow: BorrowRef<'b>,
+}
+
+impl<T> Deref for BqlRef<'_, T> {
+ type Target = T;
+
+ #[inline]
+ fn deref(&self) -> &T {
+ // SAFETY: the value is accessible as long as we hold our borrow.
+ unsafe { self.value.as_ref() }
+ }
+}
+
+impl<'b, T> BqlRef<'b, T> {
+ /// Copies a `BqlRef`.
+ ///
+ /// The `BqlRefCell` is already immutably borrowed, so this cannot fail.
+ ///
+ /// This is an associated function that needs to be used as
+ /// `BqlRef::clone(...)`. A `Clone` implementation or a method would
+ /// interfere with the widespread use of `r.borrow().clone()` to clone
+ /// the contents of a `BqlRefCell`.
+ #[must_use]
+ #[inline]
+ #[allow(clippy::should_implement_trait)]
+ pub fn clone(orig: &BqlRef<'b, T>) -> BqlRef<'b, T> {
+ BqlRef {
+ value: orig.value,
+ borrow: orig.borrow.clone(),
+ }
+ }
+}
+
+impl<T: fmt::Debug> fmt::Debug for BqlRef<'_, T> {
+ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+ (**self).fmt(f)
+ }
+}
+
+impl<T: fmt::Display> fmt::Display for BqlRef<'_, T> {
+ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+ (**self).fmt(f)
+ }
+}
+
+struct BorrowRefMut<'b> {
+ borrow: &'b Cell<BorrowFlag>,
+}
+
+impl<'b> BorrowRefMut<'b> {
+ #[inline]
+ fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
+ // There must currently be no existing references when borrow_mut() is
+ // called, so we explicitly only allow going from UNUSED to UNUSED - 1.
+ match borrow.get() {
+ UNUSED => {
+ borrow.set(UNUSED - 1);
+ Some(BorrowRefMut { borrow })
+ }
+ _ => None,
+ }
+ }
+}
+
+impl Drop for BorrowRefMut<'_> {
+ #[inline]
+ fn drop(&mut self) {
+ let borrow = self.borrow.get();
+ debug_assert!(is_writing(borrow));
+ self.borrow.set(borrow + 1);
+ bql_block_unlock(false)
+ }
+}
+
+/// A wrapper type for a mutably borrowed value from a `BqlRefCell<T>`.
+///
+/// See the [module-level documentation](self) for more.
+pub struct BqlRefMut<'b, T: 'b> {
+ // NB: we use a pointer instead of `&'b mut T` to avoid `noalias` violations, because a
+ // `BqlRefMut` argument doesn't hold exclusivity for its whole scope, only until it drops.
+ value: NonNull<T>,
+ _borrow: BorrowRefMut<'b>,
+ // `NonNull` is covariant over `T`, so we need to reintroduce invariance.
+ marker: PhantomData<&'b mut T>,
+}
+
+impl<T> Deref for BqlRefMut<'_, T> {
+ type Target = T;
+
+ #[inline]
+ fn deref(&self) -> &T {
+ // SAFETY: the value is accessible as long as we hold our borrow.
+ unsafe { self.value.as_ref() }
+ }
+}
+
+impl<T> DerefMut for BqlRefMut<'_, T> {
+ #[inline]
+ fn deref_mut(&mut self) -> &mut T {
+ // SAFETY: the value is accessible as long as we hold our borrow.
+ unsafe { self.value.as_mut() }
+ }
+}
+
+impl<T: fmt::Debug> fmt::Debug for BqlRefMut<'_, T> {
+ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+ (**self).fmt(f)
+ }
+}
+
+impl<T: fmt::Display> fmt::Display for BqlRefMut<'_, T> {
+ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+ (**self).fmt(f)
+ }
+}
projects / qemu.git / commitdiff