Make it possible to have Rust code call into tracepoints defined by C
code. It is still required that the tracepoint is declared in a C
header, and that this header is included in the input to bindgen.

Instead of calling __DO_TRACE directly, the exported rust_do_trace_
function calls an inline helper function. This is because the `cond`
argument does not exist at the callsite of DEFINE_RUST_DO_TRACE.

__DECLARE_TRACE always emits an inline static and an extern declaration
that is only used when CREATE_RUST_TRACE_POINTS is set. These should not
end up in the final binary so it is not a problem that they sometimes
are emitted without a user.

Reviewed-by: Carlos Llamas <cmllamas@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 include/linux/tracepoint.h      | 28 ++++++++++++++++++++++-
 include/trace/define_trace.h    | 12 ++++++++++
 rust/bindings/bindings_helper.h |  1 +
 rust/kernel/lib.rs              |  1 +
 rust/kernel/tracepoint.rs       | 49 +++++++++++++++++++++++++++++++++++++++++
 5 files changed, 90 insertions(+), 1 deletion(-)

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index 0dc67fad706c..84c4924e499f 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -225,6 +225,18 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
 			preempt_enable_notrace();			\
 	} while (0)
 
+/*
+ * Declare an exported function that Rust code can call to trigger this
+ * tracepoint. This function does not include the static branch; that is done
+ * in Rust to avoid a function call when the tracepoint is disabled.
+ */
+#define DEFINE_RUST_DO_TRACE(name, proto, args)
+#define __DEFINE_RUST_DO_TRACE(name, proto, args)			\
+	notrace void rust_do_trace_##name(proto)			\
+	{								\
+		__rust_do_trace_##name(args);				\
+	}
+
 /*
  * Make sure the alignment of the structure in the __tracepoints section will
  * not add unwanted padding between the beginning of the section and the
@@ -240,6 +252,7 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
 	extern int __traceiter_##name(data_proto);			\
 	DECLARE_STATIC_CALL(tp_func_##name, __traceiter_##name);	\
 	extern struct tracepoint __tracepoint_##name;			\
+	extern void rust_do_trace_##name(proto);			\
 	static inline int						\
 	register_trace_##name(void (*probe)(data_proto), void *data)	\
 	{								\
@@ -271,6 +284,12 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
 
 #define __DECLARE_TRACE(name, proto, args, cond, data_proto)		\
 	__DECLARE_TRACE_COMMON(name, PARAMS(proto), PARAMS(args), cond, PARAMS(data_proto)) \
+	static inline void __rust_do_trace_##name(proto)		\
+	{								\
+		__DO_TRACE(name,					\
+			TP_ARGS(args),					\
+			TP_CONDITION(cond), 0);				\
+	}								\
 	static inline void trace_##name(proto)				\
 	{								\
 		if (static_branch_unlikely(&__tracepoint_##name.key))	\
@@ -285,6 +304,12 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
 
 #define __DECLARE_TRACE_SYSCALL(name, proto, args, cond, data_proto)	\
 	__DECLARE_TRACE_COMMON(name, PARAMS(proto), PARAMS(args), cond, PARAMS(data_proto)) \
+	static inline void __rust_do_trace_##name(proto)		\
+	{								\
+		__DO_TRACE(name,					\
+			TP_ARGS(args),					\
+			TP_CONDITION(cond), 1);				\
+	}								\
 	static inline void trace_##name(proto)				\
 	{								\
 		if (static_branch_unlikely(&__tracepoint_##name.key))	\
@@ -339,7 +364,8 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
 	void __probestub_##_name(void *__data, proto)			\
 	{								\
 	}								\
-	DEFINE_STATIC_CALL(tp_func_##_name, __traceiter_##_name);
+	DEFINE_STATIC_CALL(tp_func_##_name, __traceiter_##_name);	\
+	DEFINE_RUST_DO_TRACE(_name, TP_PROTO(proto), TP_ARGS(args))
 
 #define DEFINE_TRACE(name, proto, args)		\
 	DEFINE_TRACE_FN(name, NULL, NULL, PARAMS(proto), PARAMS(args));
diff --git a/include/trace/define_trace.h b/include/trace/define_trace.h
index ff5fa17a6259..0557626b6f6a 100644
--- a/include/trace/define_trace.h
+++ b/include/trace/define_trace.h
@@ -76,6 +76,13 @@
 #define DECLARE_TRACE(name, proto, args)	\
 	DEFINE_TRACE(name, PARAMS(proto), PARAMS(args))
 
+/* If requested, create helpers for calling these tracepoints from Rust. */
+#ifdef CREATE_RUST_TRACE_POINTS
+#undef DEFINE_RUST_DO_TRACE
+#define DEFINE_RUST_DO_TRACE(name, proto, args)	\
+	__DEFINE_RUST_DO_TRACE(name, PARAMS(proto), PARAMS(args))
+#endif
+
 #undef TRACE_INCLUDE
 #undef __TRACE_INCLUDE
 
@@ -134,6 +141,11 @@
 # undef UNDEF_TRACE_INCLUDE_PATH
 #endif
 
+#ifdef CREATE_RUST_TRACE_POINTS
+# undef DEFINE_RUST_DO_TRACE
+# define DEFINE_RUST_DO_TRACE(name, proto, args)
+#endif
+
 /* We may be processing more files */
 #define CREATE_TRACE_POINTS
 
diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index e0846e7e93e6..752572e638a6 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -20,6 +20,7 @@
 #include <linux/refcount.h>
 #include <linux/sched.h>
 #include <linux/slab.h>
+#include <linux/tracepoint.h>
 #include <linux/wait.h>
 #include <linux/workqueue.h>
 
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 708ff817ccc3..55f81f49024e 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -54,6 +54,7 @@
 pub mod sync;
 pub mod task;
 pub mod time;
+pub mod tracepoint;
 pub mod types;
 pub mod uaccess;
 pub mod workqueue;
diff --git a/rust/kernel/tracepoint.rs b/rust/kernel/tracepoint.rs
new file mode 100644
index 000000000000..c6e80aa99e8e
--- /dev/null
+++ b/rust/kernel/tracepoint.rs
@@ -0,0 +1,49 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024 Google LLC.
+
+//! Logic for tracepoints.
+
+/// Declare the Rust entry point for a tracepoint.
+///
+/// This macro generates an unsafe function that calls into C, and its safety requirements will be
+/// whatever the relevant C code requires. To document these safety requirements, you may add
+/// doc-comments when invoking the macro.
+#[macro_export]
+macro_rules! declare_trace {
+    ($($(#[$attr:meta])* $pub:vis unsafe fn $name:ident($($argname:ident : $argtyp:ty),* $(,)?);)*) => {$(
+        $( #[$attr] )*
+        #[inline(always)]
+        $pub unsafe fn $name($($argname : $argtyp),*) {
+            #[cfg(CONFIG_TRACEPOINTS)]
+            {
+                // SAFETY: It's always okay to query the static key for a tracepoint.
+                let should_trace = unsafe {
+                    $crate::macros::paste! {
+                        $crate::jump_label::static_branch_unlikely!(
+                            $crate::bindings::[< __tracepoint_ $name >],
+                            $crate::bindings::tracepoint,
+                            key
+                        )
+                    }
+                };
+
+                if should_trace {
+                    $crate::macros::paste! {
+                        // SAFETY: The caller guarantees that it is okay to call this tracepoint.
+                        unsafe { $crate::bindings::[< rust_do_trace_ $name >]($($argname),*) };
+                    }
+                }
+            }
+
+            #[cfg(not(CONFIG_TRACEPOINTS))]
+            {
+                // If tracepoints are disabled, insert a trivial use of each argument
+                // to avoid unused argument warnings.
+                $( let _unused = $argname; )*
+            }
+        }
+    )*}
+}
+
+pub use declare_trace;

-- 
2.47.0.rc1.288.g06298d1525-goog