[PATCH v4 0/2] Tracepoints and static branch in Rust

[Alice Ryhl]

An important part of a production ready Linux kernel driver is
tracepoints. So to write production ready Linux kernel drivers in Rust,
we must be able to call tracepoints from Rust code. This patch series
adds support for calling tracepoints declared in C from Rust.

To use the tracepoint support, you must:

1. Declare the tracepoint in a C header file as usual.

2. Add #define CREATE_RUST_TRACE_POINTS next to your
   #define CREATE_TRACE_POINTS.

2. Make sure that the header file is visible to bindgen.

3. Use the declare_trace! macro in your Rust code to generate Rust
   functions that call into the tracepoint.

For example, the kernel has a tracepoint called `sched_kthread_stop`. It
is declared like this:

	TRACE_EVENT(sched_kthread_stop,
		TP_PROTO(struct task_struct *t),
		TP_ARGS(t),
		TP_STRUCT__entry(
			__array(	char,	comm,	TASK_COMM_LEN	)
			__field(	pid_t,	pid			)
		),
		TP_fast_assign(
			memcpy(__entry->comm, t->comm, TASK_COMM_LEN);
			__entry->pid	= t->pid;
		),
		TP_printk("comm=%s pid=%d", __entry->comm, __entry->pid)
	);

To call the above tracepoint from Rust code, you must first ensure that
the Rust helper for the tracepoint is generated. To do this, you would
modify kernel/sched/core.c by adding #define CREATE_RUST_TRACE_POINTS.

Next, you would include include/trace/events/sched.h in
rust/bindings/bindings_helper.h so that the exported C functions are
visible to Rust, and then you would declare the tracepoint in Rust:

	declare_trace! {
	    fn sched_kthread_stop(task: *mut task_struct);
	}

This will define an inline Rust function that checks the static key,
calling into rust_do_trace_##name if the tracepoint is active. Since
these tracepoints often take raw pointers as arguments, it may be
convenient to wrap it in a safe wrapper:

	mod raw {
	    declare_trace! {
	        fn sched_kthread_stop(task: *mut task_struct);
	    }
	}
	
	#[inline]
	pub fn trace_sched_kthread_stop(task: &Task) {
	    // SAFETY: The pointer to `task` is valid.
	    unsafe { raw::sched_kthread_stop(task.as_raw()) }
	}

A future expansion of the tracepoint support could generate these safe
versions automatically, but that is left as future work for now.

This is intended for use in the Rust Binder driver, which was originally
sent as an RFC [1]. The RFC did not include tracepoint support, but you
can see how it will be used in Rust Binder at [2]. The author has
verified that the tracepoint support works on Android devices.

This implementation implements support for static keys in Rust so that
the actual static branch happens in the Rust object file. However, the
__DO_TRACE body remains in C code. See v1 for an implementation where
__DO_TRACE is also implemented in Rust.

Link: https://lore.kernel.org/rust-for-linux/20231101-rust-binder-v1-0-08ba9197f637@google.com/ [1]
Link: https://r.android.com/3119993 [2]
Signed-off-by: Alice Ryhl <aliceryhl at google.com>
---
Changes in v4:
- Move arch-specific code into rust/kernel/arch.
- Restore DEFINE_RUST_DO_TRACE at end of define_trace.h
- Link to v3: https://lore.kernel.org/r/20240621-tracepoint-v3-0-9e44eeea2b85@google.com

Changes in v3:
- Support for Rust static_key on loongarch64 and riscv64.
- Avoid failing compilation on architectures that are missing Rust
  static_key support when the archtectures does not actually use it.
- Link to v2: https://lore.kernel.org/r/20240610-tracepoint-v2-0-faebad81b355@google.com

Changes in v2:
- Call into C code for __DO_TRACE.
- Drop static_call patch, as it is no longer needed.
- Link to v1: https://lore.kernel.org/r/20240606-tracepoint-v1-0-6551627bf51b@google.com

---
Alice Ryhl (2):
      rust: add static_key_false
      rust: add tracepoint support

 include/linux/tracepoint.h               | 18 +++++++++++-
 include/trace/define_trace.h             | 12 ++++++++
 rust/bindings/bindings_helper.h          |  1 +
 rust/kernel/arch/arm64/jump_label.rs     | 34 +++++++++++++++++++++++
 rust/kernel/arch/loongarch/jump_label.rs | 35 ++++++++++++++++++++++++
 rust/kernel/arch/mod.rs                  | 24 ++++++++++++++++
 rust/kernel/arch/riscv/jump_label.rs     | 38 ++++++++++++++++++++++++++
 rust/kernel/arch/x86/jump_label.rs       | 35 ++++++++++++++++++++++++
 rust/kernel/lib.rs                       |  3 ++
 rust/kernel/static_key.rs                | 32 ++++++++++++++++++++++
 rust/kernel/tracepoint.rs                | 47 ++++++++++++++++++++++++++++++++
 scripts/Makefile.build                   |  2 +-
 12 files changed, 279 insertions(+), 2 deletions(-)
---
base-commit: 1613e604df0cd359cf2a7fbd9be7a0bcfacfabd0
change-id: 20240606-tracepoint-31e15b90e471

Best regards,
-- 
Alice Ryhl <aliceryhl at google.com>