[PATCH v4 0/9] rust: Add Per-CPU Variable API

[Mitchell Levy <levymitchell0@gmail.com>]

This series adds an API for declaring and using per-CPU variables from
Rust, and it also adds support for Rust access to C per-CPU variables
(subject to some soundness requirements). It also adds a small sample
module, samples/rust/rust_percpu.rs, in the vein of lib/percpu_test.c.

---
Signed-off-by: Mitchell Levy <levymitchell0@gmail.com>

---
Changes in v4:
- Split what was the first patch into three patches (Thanks Yury Norov)
- Add intra-doc links to rustdoc comments (Thanks Miguel Ojeda)
- `get_remote_ptr` is no longer unsafe (Thanks Miguel Ojeda)
- Renamed the CPU mask getters to be more clear (Thanks Yury Norov)
- Add an `ExternStaticPerCpuSymbol` type for static per-CPU variables
  shared with C in order to make the extra soundness requirements for
  these variables more explicit.
- Properly drop the contents of dynamically allocated per-CPUs when the
  `DynamicPerCpu` is dropped, and properly document that
  `PerCpuAllocation` does not handle dropping values in the allocation.
- Functions on the numeric token types are now `#[inline]`
- Safety requirements of `PerCpuPtr` functions now include the fact that
  the pointed-to allocation must be live and appropriately laid out in
  memory, and SAFETY comments of uses of these functions have been
  updated to reflect these requirements.
- Included the cpumask patches first, since it might make sense for
  these to be merged separately.
- Documentation improvements throughout.
- Link to v3: https://lore.kernel.org/r/20250828-rust-percpu-v3-0-4dd92e1e7904@gmail.com

Changes in v3:
- Add a `CheckedPerCpuToken` that enables usage of per-CPU variables via
  a `&T`, allowing for a wholly safe interface when `T` allows for
  interior mutability (Thanks Benno Lossin)
- Add support for non-zeroable types to be used in a `DynamicPerCpu`.
- Remove necessity for `unsafe` to get a `StaticPerCpu` from its
  declaration (Thanks Benno Lossin)
- Allow the declaration of static per-CPU variables of types that are
  `!Sync`.
- Implement `PerCpuPtr` in terms of `MaybeUninit<T>` rather than `T` so
  as to keep all invariants in the `DynamicPerCpu` and `StaticPerCpu`
  types --- this would also enable `PerCpuPtr` to be used in a per-CPU
  type that does lazy initialization.
- Link to v2: https://lore.kernel.org/r/20250712-rust-percpu-v2-0-826f2567521b@gmail.com

Changes in v2:
- Fix kernel test robot issues
- Fix documentation error
- Require `T: Zeroable` in the dynamic case
- Link to v1: https://lore.kernel.org/r/20250624-rust-percpu-v1-0-9c59b07d2a9c@gmail.com

Changes in v1:
- Use wrapping_add in `PerCpuPtr::get_ref` since overflow is expected.
- Separate the dynamic and static cases, with shared logic in a
  `PerCpuPtr` type.
- Implement pin-hole optimizations for numeric types
- Don't assume `GFP_KERNEL` when allocating the `Arc` in the dynamic
  case.
- Link to RFC v2: https://lore.kernel.org/r/20250414-rust-percpu-v2-0-5ea0d0de13a5@gmail.com

Changes in RFC v2:
- Renamed PerCpuVariable to StaticPerCpuSymbol to be more descriptive
- Support dynamically allocated per-CPU variables via the
  PerCpuAllocation type. Rework statically allocated variables to use
  this new type.
- Make use of a token/closure-based API via the PerCpu and PerCpuToken
  types, rather than an API based on PerCpuRef that automatically
  Deref(Mut)'s into a &(mut) T.
- Rebased
- Link to RFC: https://lore.kernel.org/r/20241219-rust-percpu-v1-0-209117e822b1@gmail.com

---
Mitchell Levy (9):
      rust: cpumask: Add a `Cpumask` iterator
      rust: cpumask: Add getters for globally defined cpumasks
      rust: percpu: Add C bindings for per-CPU variable API
      rust: percpu: introduce a rust API for static per-CPU variables
      rust: percpu: introduce a rust API for dynamic per-CPU variables
      rust: percpu: add a rust per-CPU variable sample
      rust: percpu: Support non-zeroable types for DynamicPerCpu
      rust: percpu: Add pin-hole optimizations for numerics
      rust: percpu: cache per-CPU pointers in the dynamic case

 rust/helpers/cpumask.c          |   5 +
 rust/helpers/helpers.c          |   2 +
 rust/helpers/percpu.c           |  20 +++
 rust/helpers/preempt.c          |  14 ++
 rust/kernel/cpumask.rs          |  94 ++++++++++++-
 rust/kernel/lib.rs              |   3 +
 rust/kernel/percpu.rs           | 273 +++++++++++++++++++++++++++++++++++++
 rust/kernel/percpu/cpu_guard.rs |  36 +++++
 rust/kernel/percpu/dynamic.rs   | 184 +++++++++++++++++++++++++
 rust/kernel/percpu/numeric.rs   | 138 +++++++++++++++++++
 rust/kernel/percpu/static_.rs   | 218 +++++++++++++++++++++++++++++
 samples/rust/Kconfig            |   9 ++
 samples/rust/Makefile           |   1 +
 samples/rust/rust_percpu.rs     | 294 ++++++++++++++++++++++++++++++++++++++++
 14 files changed, 1290 insertions(+), 1 deletion(-)
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base-commit: f3f6b3664302e16ef1c6b91034a72df5564d6b8a
change-id: 20240813-rust-percpu-ea2f54b5da33

Best regards,
-- 
Mitchell Levy <levymitchell0@gmail.com>