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Mon, 14 Apr 2025 12:28:55 -0700 (PDT) From: Mitchell Levy Date: Mon, 14 Apr 2025 12:28:27 -0700 Subject: [PATCH RFC v2 1/3] rust: percpu: introduce a rust API for per-CPU variables MIME-Version: 1.0 Message-Id: <20250414-rust-percpu-v2-1-5ea0d0de13a5@gmail.com> References: <20250414-rust-percpu-v2-0-5ea0d0de13a5@gmail.com> In-Reply-To: <20250414-rust-percpu-v2-0-5ea0d0de13a5@gmail.com> To: Miguel Ojeda , Alex Gaynor , Boqun Feng , Gary Guo , =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= , Benno Lossin , Andreas Hindborg , Alice Ryhl , Trevor Gross , Andrew Morton , Dennis Zhou , Tejun Heo , Christoph Lameter , Danilo Krummrich Cc: linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org, linux-mm@kvack.org, Mitchell Levy X-Mailer: b4 0.14.0 X-Developer-Signature: v=1; a=ed25519-sha256; t=1744658935; l=13401; i=levymitchell0@gmail.com; s=20240719; h=from:subject:message-id; bh=nIqel8sF5aGOeh5Zf3p9yAVGtxyjMWnYFGuVyV7v7Ew=; b=vtj/4I/d40UsQeU/uI4V3EplUz1GsyP/yr1B8Vgqs8WIn8iPCtAwX4Vpq9mUNfcz8dklRhEtU Xg+yT3wyreEAwZwIxFAIz1oBDlHanTgy6ghc8aoRTddN/djbgBn0T3n X-Developer-Key: i=levymitchell0@gmail.com; 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Add a PerCpuAllocation type used to track and, in the case of dynamic allocations, create and free per-CPU allocations. Add a define_per_cpu! macro to create static per-CPU allocations. Add a PerCpu type that manages PerCpuAllocation's being used from multiple cores, and add a PerCpuToken type used to assert exclusive access to a particular per-CPU variable. Co-developed-by: Boqun Feng Signed-off-by: Boqun Feng Signed-off-by: Mitchell Levy --- rust/helpers/helpers.c | 2 + rust/helpers/percpu.c | 9 ++ rust/helpers/preempt.c | 14 +++ rust/kernel/lib.rs | 3 + rust/kernel/percpu.rs | 239 ++++++++++++++++++++++++++++++++++++++++ rust/kernel/percpu/cpu_guard.rs | 29 +++++ 6 files changed, 296 insertions(+) diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c index e1c21eba9b15..cff605710f77 100644 --- a/rust/helpers/helpers.c +++ b/rust/helpers/helpers.c @@ -23,7 +23,9 @@ #include "page.c" #include "platform.c" #include "pci.c" +#include "percpu.c" #include "pid_namespace.c" +#include "preempt.c" #include "rbtree.c" #include "rcu.c" #include "refcount.c" diff --git a/rust/helpers/percpu.c b/rust/helpers/percpu.c new file mode 100644 index 000000000000..a091389f730f --- /dev/null +++ b/rust/helpers/percpu.c @@ -0,0 +1,9 @@ +// SPDX-License-Identifier: GPL-2.0 + +#include + +void __percpu *rust_helper_alloc_percpu(size_t sz, size_t align) +{ + return __alloc_percpu(sz, align); +} + diff --git a/rust/helpers/preempt.c b/rust/helpers/preempt.c new file mode 100644 index 000000000000..2c7529528ddd --- /dev/null +++ b/rust/helpers/preempt.c @@ -0,0 +1,14 @@ +// SPDX-License-Identifier: GPL-2.0 + +#include + +void rust_helper_preempt_disable(void) +{ + preempt_disable(); +} + +void rust_helper_preempt_enable(void) +{ + preempt_enable(); +} + diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs index de07aadd1ff5..dc88becdb338 100644 --- a/rust/kernel/lib.rs +++ b/rust/kernel/lib.rs @@ -67,6 +67,9 @@ pub mod page; #[cfg(CONFIG_PCI)] pub mod pci; +// Only x86_64 is supported by percpu for now +#[cfg(CONFIG_X86_64)] +pub mod percpu; pub mod pid_namespace; pub mod platform; pub mod prelude; diff --git a/rust/kernel/percpu.rs b/rust/kernel/percpu.rs new file mode 100644 index 000000000000..ecce8cdaa22a --- /dev/null +++ b/rust/kernel/percpu.rs @@ -0,0 +1,239 @@ +// SPDX-License-Identifier: GPL-2.0 +//! This module contains abstractions for creating and using per-CPU variables from Rust. +//! See the define_per_cpu! macro and the PerCpu type. +pub mod cpu_guard; + +use bindings::{alloc_percpu, free_percpu}; + +use crate::percpu::cpu_guard::CpuGuard; +use crate::prelude::*; +use crate::sync::Arc; + +use core::arch::asm; +use core::marker::PhantomData; + +use ffi::c_void; + +/// Represents an allocation of a per-CPU variable via alloc_percpu or a static per-CPU +/// allocation. Calls free_percpu when dropped if not a static allocation. +pub struct PerCpuAllocation { + /// Offset into the per-CPU area: either the address of a statically-allocated per-CPU variable + /// or a "pointer" returned by alloc_percpu or similar. + offset: usize, + /// Whether `self` is statically allocated + is_static: bool, + /// PhantomData so the compiler doesn't complain about `T` being unused. + deref_type: PhantomData, +} + +/// Holds a per-CPU variable. +pub struct PerCpu { + alloc: Arc>, +} + +/// Represents exclusive access to the memory location pointed at by a particular PerCpu. +pub struct PerCpuToken<'a, T> { + _guard: CpuGuard, + pcpu: &'a mut PerCpu, +} + +/// A wrapper used for declaring static per-CPU variables. These symbols are "virtual" in that the +/// linker uses them to generate offsets into each cpu's per-cpu area, but shouldn't be read +/// from/written to directly. The fact that the statics are immutable prevents them being written +/// to (generally), this struct having _val be non-public prevents reading from them. +/// +/// The end-user of the per-CPU API should make use of the define_per_cpu! macro instead of +/// declaring variables of this type directly. +#[repr(transparent)] +pub struct StaticPerCpuSymbol { + _val: T, // generate a correctly sized type +} + +impl PerCpuAllocation { + /// Dynamically allocates a space in the per-CPU area suitably sized and aligned to hold a `T`. + /// + /// Returns `None` under the same circumstances the C function `alloc_percpu` returns `NULL`. + pub fn new() -> Option> { + // SAFETY: No preconditions to call alloc_percpu + let ptr: *mut c_void = unsafe { alloc_percpu(size_of::(), align_of::()) }; + if ptr.is_null() { + return None; + } + + Some(Self { + offset: ptr as usize, + is_static: false, + deref_type: PhantomData, + }) + } + + /// Convert a statically allocated per-CPU variable (via its offset, i.e., the address of the + /// declared symbol) into a `PerCpuAllocation`. + /// + /// # Safety + /// `offset` must be a valid offset into the per-CPU area that's suitably sized and aligned to + /// hold a `T`. + pub unsafe fn new_static(offset: usize) -> PerCpuAllocation { + Self { + offset, + is_static: true, + deref_type: PhantomData, + } + } +} + +impl Drop for PerCpuAllocation { + fn drop(&mut self) { + if !self.is_static { + // SAFETY: self.offset was returned by alloc_percpu, and so was a valid pointer into + // the percpu area, and has remained valid by the invariants of PerCpuAllocation. + unsafe { free_percpu(self.offset as *mut c_void) } + } + } +} + +impl PerCpu { + /// Allocates a new per-CPU variable + pub fn new() -> Option { + let alloc: PerCpuAllocation = PerCpuAllocation::new()?; + + let arc = Arc::new(alloc, GFP_KERNEL).ok()?; + + Some(Self { alloc: arc }) + } + + /// Wraps a `PerCpuAllocation` in a `PerCpu` + pub fn new_from_allocation(alloc: PerCpuAllocation) -> Option { + let arc = Arc::new(alloc, GFP_KERNEL).ok()?; + Some(Self { alloc: arc }) + } + + /// Creates a new PerCpu pointing to the same underlying variable + pub fn clone(&self) -> Self { + Self { + alloc: self.alloc.clone(), + } + } + + /// Get a `&mut T` to the per-CPU variable represented by `&mut self` + /// + /// # Safety + /// The returned `&mut T` must follow Rust's aliasing rules. That is, no other `&(mut) T` may + /// exist that points to the same location in memory. In practice, this means that any PerCpu + /// holding a reference to the same PerCpuAllocation as `self` mut not call `get_ref` for as + /// long as the returned reference lives. + unsafe fn get_ref(&mut self) -> &mut T { + // SAFETY: addr_of!(this_cpu_off) is guaranteed to be a valid per-CPU offset by the per-CPU + // subsystem + let this_cpu_off_pcpu: PerCpuAllocation = + unsafe { PerCpuAllocation::new_static(core::ptr::addr_of!(this_cpu_off) as usize) }; + let mut this_cpu_area: *mut c_void; + // SAFETY: gs + this_cpu_off_pcpu.offset is guaranteed to be a valid pointer because `gs` + // points to the per-CPU area and this_cpu_off_pcpu is a valid per-CPU allocation. + unsafe { + asm!( + // For some reason, the asm! parser doesn't like + // mov {out}, [gs:{off_val}] + // so we use the less intuitive prefix version instead + "gs mov {out}, [{off_val}]", + off_val = in(reg) this_cpu_off_pcpu.offset, + out = out(reg) this_cpu_area, + ) + }; + // SAFETY: this_cpu_area + self.alloc.offset is guaranteed to be a valid pointer by the + // per-CPU subsystem and the invariant that self.offset is a valid offset into the per-CPU + // area. + // + // We have exclusive access to self via &mut self, so we know no-one else has a reference + // to the underlying pcpu variable because of the safety requirements of this function. + unsafe { &mut *((this_cpu_area.add(self.alloc.offset)) as *mut T) } + } + + /// Produces a token, asserting that the holder has exclusive access to the underlying memory + /// pointed to by `self` + /// + /// # Safety + /// `func` (or its callees that execute on the same CPU) may not call `get_ref` on another + /// `PerCpu` that represents the same per-CPU variable as `&mut self` (that is, they must + /// not be `clone()`s of each other or, in the case of statically allocated variables, + /// additionally can't both have come from the same `define_per_cpu!`) for the lifetime of the + /// returned token. + /// + /// In particular, this requires that the underlying per-CPU variable cannot ever be mutated + /// from an interrupt context, unless irqs are disabled for the lifetime of the returned + /// `PerCpuToken`. + pub unsafe fn get(&mut self, guard: CpuGuard) -> PerCpuToken<'_, T> { + PerCpuToken { + _guard: guard, + pcpu: self, + } + } +} + +impl PerCpuToken<'_, T> { + /// Immediately invokes `func` with a `&mut T` that points at the underlying per-CPU variable + /// that `&mut self` represents. + pub fn with(&mut self, func: U) + where + U: FnOnce(&mut T), + { + // SAFETY: The existence of a PerCpuToken means that the requirements for get_ref are + // satisfied. + func(unsafe { self.pcpu.get_ref() }); + } +} + +/// define_per_cpu! is analogous to the C DEFINE_PER_CPU macro in that it lets you create a +/// statically allocated per-CPU variable. +/// +/// # Example +/// ``` +/// use kernel::define_per_cpu; +/// use kernel::percpu::StaticPerCpuSymbol; +/// +/// define_per_cpu!(pub MY_PERCPU: u64 = 0); +/// ``` +#[macro_export] +macro_rules! define_per_cpu { + ($vis:vis $id:ident: $ty:ty = $expr:expr) => { + $crate::macros::paste! { + // Expand $expr outside of the unsafe block to avoid silently allowing unsafe code to be + // used without a user-facing unsafe block + static [<__INIT_ $id>]: $ty = $expr; + + // SAFETY: StaticPerCpuSymbol is #[repr(transparent)], so we can freely convert from T + #[link_section = ".data..percpu"] + $vis static $id: StaticPerCpuSymbol<$ty> = unsafe { + core::mem::transmute::<$ty, StaticPerCpuSymbol<$ty>>([<__INIT_ $id>]) + }; + } + }; +} + +/// Gets a `PerCpu` from a symbol declared with `define_per_cpu!` or `declare_extern_per_cpu!`. +/// +/// # Safety +/// `$id` must be declared with either `define_per_cpu!` or `declare_extern_per_cpu!`, and the +/// returned value must be stored in a `PerCpu` where `T` matches the declared type of `$id`. +#[macro_export] +macro_rules! unsafe_get_per_cpu { + ($id:ident, $guard:expr) => {{ + let off = core::ptr::addr_of!($id); + $crate::percpu::PerCpu::new_from_allocation($crate::percpu::PerCpuAllocation::new_static( + off as usize, + )) + }}; +} + +/// Declares a StaticPerCpuSymbol corresponding to a per-CPU variable defined in C. Be sure to read +/// the safety requirements of `PerCpu::get`. +#[macro_export] +macro_rules! declare_extern_per_cpu { + ($id:ident: $ty:ty) => { + extern "C" { + static $id: StaticPerCpuSymbol<$ty>; + } + }; +} + +declare_extern_per_cpu!(this_cpu_off: u64); diff --git a/rust/kernel/percpu/cpu_guard.rs b/rust/kernel/percpu/cpu_guard.rs new file mode 100644 index 000000000000..6f7d320a5c9a --- /dev/null +++ b/rust/kernel/percpu/cpu_guard.rs @@ -0,0 +1,29 @@ +// SPDX-License-Identifier: GPL-2.0 +//! Contains abstractions for disabling CPU preemption. See CpuGuard. + +/// A RAII guard for bindings::preempt_disable and bindings::preempt_enable. Guarantees preemption +/// is disabled for as long as this object exists. +pub struct CpuGuard { + // Don't make one without using new() + _phantom: (), +} + +impl CpuGuard { + /// Create a new CpuGuard. Disables preemption for its lifetime. + pub fn new() -> Self { + // SAFETY: There are no preconditions required to call preempt_disable + unsafe { + bindings::preempt_disable(); + } + CpuGuard { _phantom: () } + } +} + +impl Drop for CpuGuard { + fn drop(&mut self) { + // SAFETY: There are no preconditions required to call preempt_enable + unsafe { + bindings::preempt_enable(); + } + } +}