[PATCH v2 1/2] rust: add static_key_false

[Alice Ryhl]

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.

It is not possible to use the existing C implementation of
arch_static_branch because it passes the argument `key` to inline
assembly as an 'i' parameter, so any attempt to add a C helper for this
function will fail to compile because the value of `key` must be known
at compile-time.

Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
---
 rust/kernel/lib.rs        |  1 +
 rust/kernel/static_key.rs | 87 +++++++++++++++++++++++++++++++++++++++++++++++
 scripts/Makefile.build    |  2 +-
 3 files changed, 89 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index fbd91a48ff8b..a0be9544996d 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -38,6 +38,7 @@
 pub mod prelude;
 pub mod print;
 mod static_assert;
+pub mod static_key;
 #[doc(hidden)]
 pub mod std_vendor;
 pub mod str;
diff --git a/rust/kernel/static_key.rs b/rust/kernel/static_key.rs
new file mode 100644
index 000000000000..6c3dbe14c98a
--- /dev/null
+++ b/rust/kernel/static_key.rs
@@ -0,0 +1,87 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024 Google LLC.
+
+//! Logic for static keys.
+
+use crate::bindings::*;
+
+#[doc(hidden)]
+#[macro_export]
+#[cfg(target_arch = "x86_64")]
+macro_rules! _static_key_false {
+    ($key:path, $keytyp:ty, $field:ident) => {'my_label: {
+        core::arch::asm!(
+            r#"
+            1: .byte 0x0f,0x1f,0x44,0x00,0x00
+
+            .pushsection __jump_table,  "aw"
+            .balign 8
+            .long 1b - .
+            .long {0} - .
+            .quad {1} + {2} - .
+            .popsection
+            "#,
+            label {
+                break 'my_label true;
+            },
+            sym $key,
+            const ::core::mem::offset_of!($keytyp, $field),
+        );
+
+        break 'my_label false;
+    }};
+}
+
+#[doc(hidden)]
+#[macro_export]
+#[cfg(target_arch = "aarch64")]
+macro_rules! _static_key_false {
+    ($key:path, $keytyp:ty, $field:ident) => {'my_label: {
+        core::arch::asm!(
+            r#"
+            1: nop
+
+            .pushsection __jump_table,  "aw"
+            .align 3
+            .long 1b - ., {0} - .
+            .quad {1} + {2} - .
+            .popsection
+            "#,
+            label {
+                break 'my_label true;
+            },
+            sym $key,
+            const ::core::mem::offset_of!($keytyp, $field),
+        );
+
+        break 'my_label false;
+    }};
+}
+
+/// Branch based on a static key.
+///
+/// Takes three arguments:
+///
+/// * `key` - the path to the static variable containing the `static_key`.
+/// * `keytyp` - the type of `key`.
+/// * `field` - the name of the field of `key` that contains the `static_key`.
+#[macro_export]
+macro_rules! static_key_false {
+    // Forward to the real implementation. Separated like this so that we don't have to duplicate
+    // the documentation.
+    ($key:path, $keytyp:ty, $field:ident) => {{
+        // Assert that `$key` has type `$keytyp` and that `$key.$field` has type `static_key`.
+        //
+        // SAFETY: We know that `$key` is a static because otherwise the inline assembly will not
+        // compile. The raw pointers created in this block are in-bounds of `$key`.
+        static _TY_ASSERT: () = unsafe {
+            let key: *const $keytyp = ::core::ptr::addr_of!($key);
+            let _: *const $crate::bindings::static_key = ::core::ptr::addr_of!((*key).$field);
+        };
+
+        $crate::static_key::_static_key_false! { $key, $keytyp, $field }
+    }};
+}
+
+pub use {_static_key_false, static_key_false};
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index efacca63c897..60197c1c063f 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -263,7 +263,7 @@ $(obj)/%.lst: $(obj)/%.c FORCE
 # Compile Rust sources (.rs)
 # ---------------------------------------------------------------------------
 
-rust_allowed_features := new_uninit
+rust_allowed_features := asm_const,asm_goto,new_uninit
 
 # `--out-dir` is required to avoid temporaries being created by `rustc` in the
 # current working directory, which may be not accessible in the out-of-tree

-- 
2.45.2.505.gda0bf45e8d-goog