[PATCH v8 1/5 alt] rust: add generic static_key_false

[Alice Ryhl <aliceryhl@google.com>]

Add just enough support for static key so that we can use it from
tracepoints. Tracepoints rely on `static_key_false` even though it is
deprecated, so we add the same functionality to Rust.

This patch only provides a generic implementation without code patching
(matching the one used when CONFIG_JUMP_LABEL is disabled). Later
patches add support for inline asm implementations that use runtime
patching.

When CONFIG_JUMP_LABEL is unset, `static_key_count` is a static inline
function, so a Rust helper is defined for `static_key_count` in this
case. If Rust is compiled with LTO, this call should get inlined. The
helper can be eliminated once we have the necessary inline asm to make
atomic operations from Rust.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
This is an alternate version of patch 1 that resolves the conflict with
https://lore.kernel.org/all/20240725183325.122827-7-ojeda@kernel.org/

 rust/bindings/bindings_helper.h |  1 +
 rust/helpers/helpers.c          |  1 +
 rust/helpers/tracepoint.c       | 18 ++++++++++++++++++
 rust/kernel/jump_label.rs       | 29 +++++++++++++++++++++++++++++
 rust/kernel/lib.rs              |  1 +
 5 files changed, 50 insertions(+)
 create mode 100644 rust/helpers/tracepoint.c
 create mode 100644 rust/kernel/jump_label.rs

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index ae82e9c941af..e0846e7e93e6 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -14,6 +14,7 @@
 #include <linux/ethtool.h>
 #include <linux/firmware.h>
 #include <linux/jiffies.h>
+#include <linux/jump_label.h>
 #include <linux/mdio.h>
 #include <linux/phy.h>
 #include <linux/refcount.h>
diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
index 173533616c91..5b17839de43a 100644
--- a/rust/helpers/helpers.c
+++ b/rust/helpers/helpers.c
@@ -20,6 +20,7 @@
 #include "slab.c"
 #include "spinlock.c"
 #include "task.c"
+#include "tracepoint.c"
 #include "uaccess.c"
 #include "wait.c"
 #include "workqueue.c"
diff --git a/rust/helpers/tracepoint.c b/rust/helpers/tracepoint.c
new file mode 100644
index 000000000000..02aafb2b226f
--- /dev/null
+++ b/rust/helpers/tracepoint.c
@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Helpers for tracepoints. At the moment, helpers are only needed when
+ * CONFIG_JUMP_LABEL is disabled, as `static_key_count` is only marked inline
+ * in that case.
+ *
+ * Copyright (C) 2024 Google LLC.
+ */
+
+#include <linux/jump_label.h>
+
+#ifndef CONFIG_JUMP_LABEL
+int rust_helper_static_key_count(struct static_key *key)
+{
+	return static_key_count(key);
+}
+#endif
diff --git a/rust/kernel/jump_label.rs b/rust/kernel/jump_label.rs
new file mode 100644
index 000000000000..011e1fc1d19a
--- /dev/null
+++ b/rust/kernel/jump_label.rs
@@ -0,0 +1,29 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024 Google LLC.
+
+//! Logic for static keys.
+//!
+//! C header: [`include/linux/jump_label.h`](srctree/include/linux/jump_label.h).
+
+/// Branch based on a static key.
+///
+/// Takes three arguments:
+///
+/// * `key` - the path to the static variable containing the `static_key`.
+/// * `keytyp` - the type of `key`.
+/// * `field` - the name of the field of `key` that contains the `static_key`.
+///
+/// # Safety
+///
+/// The macro must be used with a real static key defined by C.
+#[macro_export]
+macro_rules! static_key_false {
+    ($key:path, $keytyp:ty, $field:ident) => {{
+        let _key: *const $keytyp = ::core::ptr::addr_of!($key);
+        let _key: *const $crate::bindings::static_key = ::core::ptr::addr_of!((*_key).$field);
+
+        $crate::bindings::static_key_count(_key.cast_mut()) > 0
+    }};
+}
+pub use static_key_false;
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 274bdc1b0a82..91af9f75d121 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -36,6 +36,7 @@
 pub mod firmware;
 pub mod init;
 pub mod ioctl;
+pub mod jump_label;
 #[cfg(CONFIG_KUNIT)]
 pub mod kunit;
 #[cfg(CONFIG_NET)]
-- 
2.46.0.184.g6999bdac58-goog