[PATCH RFC 0/3] rust: Add Per-CPU Variable API

[Mitchell Levy <levymitchell0@gmail.com>]

This series adds an API for declaring an 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 test
module, lib/percpu_test_rust.rs, in the vein of lib/percpu_test.c.

---
I am sending this patch as an RFC to gather feedback on the API as well
as get some input on correctness. In particular, the unsafe getter macro
is somewhat cumbersome, though its safety requirements for pure-Rust
code aren't too difficult to verify (interoperation with C code is
certainly much more tricky).

It was suggested that I base my implementation on Rust's thread-local
storage API. However, this API requires that for a thread-local T, any
particular usage must be via a &T. Thus, many times it is required that
thread-local variables be in the form RefCell<T> or Cell<T>. This has
some pretty severe disadvantages in the context of per-CPU variables.
Namely, RefCell<T> may panic if .borrow_mut is called multiple times at
different points in the call stack. This is similar to the problem
inherent in my current implementation (that is, unsafe_get_per_cpu_ref
must be used carefully so as to not create aliasing mut references), but
this implementation flags this potential problem via an unsafe block and
the resulting PerCpuRef can be easily passed along and used fairly
easily. Cell<T> on the other hand requires a copy any time the
underlying value is used, which is entirely avoided here.

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

---
Mitchell Levy (3):
      rust: percpu: introduce a rust API for per-CPU variables
      rust: rust-analyzer: add lib to dirs searched for crates
      rust: percpu: add a rust per-CPU variable test

 lib/Kconfig.debug                 |   9 +++
 lib/Makefile                      |   1 +
 lib/percpu_test_rust.rs           |  65 +++++++++++++++
 rust/helpers/helpers.c            |   1 +
 rust/helpers/preempt.c            |  14 ++++
 rust/kernel/lib.rs                |   3 +
 rust/kernel/percpu.rs             | 161 ++++++++++++++++++++++++++++++++++++++
 rust/kernel/percpu/cpu_guard.rs   |  29 +++++++
 scripts/generate_rust_analyzer.py |   2 +-
 9 files changed, 284 insertions(+), 1 deletion(-)
---
base-commit: 0c5928deada15a8d075516e6e0d9ee19011bb000
change-id: 20240813-rust-percpu-ea2f54b5da33

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