Alignment operations are very common in the kernel. Since they are always performed using a power-of-two value, enforcing this invariant through a dedicated type leads to fewer bugs and can improve the generated code.
Introduce the `Alignment` type, inspired by the nightly Rust type of the same name and providing the same interface, and a new `Alignable` trait allowing unsigned integers to be aligned up or down. Reviewed-by: Alice Ryhl <[email protected]> Reviewed-by: Danilo Krummrich <[email protected]> Signed-off-by: Alexandre Courbot <[email protected]> --- rust/kernel/lib.rs | 2 + rust/kernel/ptr.rs | 226 +++++++++++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 228 insertions(+) diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs index fcffc3988a90392f1d5fc19f15c75d9ba7104f9a..833bb070e9a6697dad93ba10c61f5dd52da70f35 100644 --- a/rust/kernel/lib.rs +++ b/rust/kernel/lib.rs @@ -17,6 +17,7 @@ // the unstable features in use. // // Stable since Rust 1.79.0. +#![feature(generic_nonzero)] #![feature(inline_const)] #![feature(pointer_is_aligned)] // @@ -111,6 +112,7 @@ pub mod platform; pub mod prelude; pub mod print; +pub mod ptr; pub mod rbtree; pub mod regulator; pub mod revocable; diff --git a/rust/kernel/ptr.rs b/rust/kernel/ptr.rs new file mode 100644 index 0000000000000000000000000000000000000000..cf419d2ec61d3850b47c4d86ff2132f13b7b60f9 --- /dev/null +++ b/rust/kernel/ptr.rs @@ -0,0 +1,226 @@ +// SPDX-License-Identifier: GPL-2.0 + +//! Types and functions to work with pointers and addresses. + +use core::fmt::Debug; +use core::mem::align_of; +use core::num::NonZero; + +use crate::build_error; + +/// Type representing an alignment, which is always a power of two. +/// +/// It is used to validate that a given value is a valid alignment, and to perform masking and +/// alignment operations. +/// +/// This is a temporary substitute for the [`Alignment`] nightly type from the standard library, +/// and to be eventually replaced by it. +/// +/// [`Alignment`]: https://github.com/rust-lang/rust/issues/102070 +/// +/// # Invariants +/// +/// An alignment is always a power of two. +#[repr(transparent)] +#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)] +pub struct Alignment(NonZero<usize>); + +impl Alignment { + /// Validates that `ALIGN` is a power of two at build-time, and returns an [`Alignment`] of the + /// same value. + /// + /// A build error is triggered if `ALIGN` is not a power of two. + /// + /// # Examples + /// + /// ``` + /// use kernel::ptr::Alignment; + /// + /// let v = Alignment::new::<16>(); + /// assert_eq!(v.as_usize(), 16); + /// ``` + #[inline(always)] + pub const fn new<const ALIGN: usize>() -> Self { + if ALIGN.is_power_of_two() { + // INVARIANT: `align` is a power of two. + // SAFETY: `align` is a power of two, and thus non-zero. + Self(unsafe { NonZero::new_unchecked(ALIGN) }) + } else { + build_error!("Provided alignment is not a power of two."); + } + } + + /// Validates that `align` is a power of two at runtime, and returns an + /// [`Alignment`] of the same value. + /// + /// Returns [`None`] if `align` is not a power of two. + /// + /// # Examples + /// + /// ``` + /// use kernel::ptr::Alignment; + /// + /// assert_eq!(Alignment::new_checked(16), Some(Alignment::new::<16>())); + /// assert_eq!(Alignment::new_checked(15), None); + /// assert_eq!(Alignment::new_checked(1), Some(Alignment::new::<1>())); + /// assert_eq!(Alignment::new_checked(0), None); + /// ``` + #[inline(always)] + pub const fn new_checked(align: usize) -> Option<Self> { + if align.is_power_of_two() { + // INVARIANT: `align` is a power of two. + // SAFETY: `align` is a power of two, and thus non-zero. + Some(Self(unsafe { NonZero::new_unchecked(align) })) + } else { + None + } + } + + /// Returns the alignment of `T`. + /// + /// This is equivalent to [`align_of`], but with the return value provided as an [`Alignment`]. + #[inline(always)] + pub const fn of<T>() -> Self { + // This cannot panic since alignments are always powers of two. + // + // We unfortunately cannot use `new` as it would require the `generic_const_exprs` feature. + const { Alignment::new_checked(align_of::<T>()).unwrap() } + } + + /// Returns this alignment as a `usize`. + /// + /// It is guaranteed to be a power of two. + /// + /// # Examples + /// + /// ``` + /// use kernel::ptr::Alignment; + /// + /// assert_eq!(Alignment::new::<16>().as_usize(), 16); + /// ``` + #[inline(always)] + pub const fn as_usize(self) -> usize { + self.as_nonzero().get() + } + + /// Returns this alignment as a [`NonZero`]. + /// + /// It is guaranteed to be a power of two. + /// + /// # Examples + /// + /// ``` + /// use kernel::ptr::Alignment; + /// + /// assert_eq!(Alignment::new::<16>().as_nonzero().get(), 16); + /// ``` + #[inline(always)] + pub const fn as_nonzero(self) -> NonZero<usize> { + // Allow the compiler to know that the value is indeed a power of two. This can help + // optimize some operations down the line, like e.g. replacing divisions by bit shifts. + if !self.0.is_power_of_two() { + // SAFETY: per the invariants, `self.0` is always a power of two so this block will + // never be reached. + unsafe { core::hint::unreachable_unchecked() } + } + self.0 + } + + /// Returns the base-2 logarithm of the alignment. + /// + /// # Examples + /// + /// ``` + /// use kernel::ptr::Alignment; + /// + /// assert_eq!(Alignment::of::<u8>().log2(), 0); + /// assert_eq!(Alignment::new::<16>().log2(), 4); + /// ``` + #[inline(always)] + pub const fn log2(self) -> u32 { + self.0.ilog2() + } + + /// Returns the mask for this alignment. + /// + /// This is equivalent to `!(self.as_usize() - 1)`. + /// + /// # Examples + /// + /// ``` + /// use kernel::ptr::Alignment; + /// + /// assert_eq!(Alignment::new::<0x10>().mask(), !0xf); + /// ``` + #[inline(always)] + pub const fn mask(self) -> usize { + // No underflow can occur as the alignment is guaranteed to be a power of two, and thus is + // non-zero. + !(self.as_usize() - 1) + } +} + +/// Trait for items that can be aligned against an [`Alignment`]. +pub trait Alignable: Sized { + /// Aligns `self` down to `alignment`. + /// + /// # Examples + /// + /// ``` + /// use kernel::ptr::{Alignable, Alignment}; + /// + /// assert_eq!(0x2f_usize.align_down(Alignment::new::<0x10>()), 0x20); + /// assert_eq!(0x30usize.align_down(Alignment::new::<0x10>()), 0x30); + /// assert_eq!(0xf0u8.align_down(Alignment::new::<0x1000>()), 0x0); + /// ``` + fn align_down(self, alignment: Alignment) -> Self; + + /// Aligns `self` up to `alignment`, returning `None` if aligning would result in an overflow. + /// + /// # Examples + /// + /// ``` + /// use kernel::ptr::{Alignable, Alignment}; + /// + /// assert_eq!(0x4fusize.align_up(Alignment::new::<0x10>()), Some(0x50)); + /// assert_eq!(0x40usize.align_up(Alignment::new::<0x10>()), Some(0x40)); + /// assert_eq!(0x0usize.align_up(Alignment::new::<0x10>()), Some(0x0)); + /// assert_eq!(u8::MAX.align_up(Alignment::new::<0x10>()), None); + /// assert_eq!(0x10u8.align_up(Alignment::new::<0x100>()), None); + /// assert_eq!(0x0u8.align_up(Alignment::new::<0x100>()), Some(0x0)); + /// ``` + fn align_up(self, alignment: Alignment) -> Option<Self>; +} + +/// Implement [`Alignable`] for unsigned integer types. +macro_rules! impl_alignable_uint { + ($($t:ty),*) => { + $( + impl Alignable for $t { + #[inline(always)] + fn align_down(self, alignment: Alignment) -> Self { + // The operands of `&` need to be of the same type so convert the alignment to + // `Self`. This means we need to compute the mask ourselves. + ::core::num::NonZero::<Self>::try_from(alignment.as_nonzero()) + .map(|align| self & !(align.get() - 1)) + // An alignment larger than `Self` always aligns down to `0`. + .unwrap_or(0) + } + + #[inline(always)] + fn align_up(self, alignment: Alignment) -> Option<Self> { + let aligned_down = self.align_down(alignment); + if self == aligned_down { + Some(aligned_down) + } else { + Self::try_from(alignment.as_usize()) + .ok() + .and_then(|align| aligned_down.checked_add(align)) + } + } + } + )* + }; +} + +impl_alignable_uint!(u8, u16, u32, u64, usize); -- 2.51.0
