rust: add static_key_false

This method is deprecated in C, but we are still adding it here because
we are mirroring how C implements tracepoints, and tracepoints use
static_key_false.

Signed-off-by: Alice Ryhl <[email protected]>

rust/kernel/lib.rs

@@ -17,6 +17,8 @@
 #![feature(new_uninit)]
 #![feature(receiver_trait)]
 #![feature(unsize)]
+#![feature(asm_const)]
+#![feature(asm_goto)]
 
 // Ensure conditional compilation based on the kernel configuration works;
 // otherwise we may silently break things like initcall handling.
@@ -39,6 +41,7 @@ pub mod prelude;
 pub mod print;
 mod static_assert;
 pub mod static_call;
+pub mod static_key;
 #[doc(hidden)]
 pub mod std_vendor;
 pub mod str;

rust/kernel/static_key.rs

@@ -0,0 +1,150 @@
+// 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")]
+#[cfg(not(CONFIG_HAVE_RUST_ASM_GOTO))]
+macro_rules! _static_key_false {
+    ($key:path, $keytyp:ty, $field:ident) => {{
+        let mut output = 1u32;
+
+        core::arch::asm!(
+            r#"
+            1: .byte 0x0f,0x1f,0x44,0x00,0x00
+
+            .pushsection __jump_table,  "aw"
+            .balign 8
+            .long 1b - .
+            .long 3f - .
+            .quad {0} + {1} - .
+            .popsection
+
+            2: mov {2:e}, 0
+            3:
+            "#,
+            sym $key,
+            const ::core::mem::offset_of!($keytyp, $field),
+            inout(reg) output,
+        );
+
+        output != 0
+    }};
+}
+
+#[doc(hidden)]
+#[macro_export]
+#[cfg(target_arch = "x86_64")]
+#[cfg(CONFIG_HAVE_RUST_ASM_GOTO)]
+macro_rules! _static_key_false {
+    ($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, $field),
+        );
+
+        break 'my_label false;
+    }};
+}
+
+#[doc(hidden)]
+#[macro_export]
+#[cfg(target_arch = "aarch64")]
+#[cfg(not(CONFIG_HAVE_RUST_ASM_GOTO))]
+macro_rules! _static_key_false {
+    ($key:path, $keytyp:ty, $field:ident) => {{
+        let mut output = 1u32;
+
+        core::arch::asm!(
+            r#"
+            1: nop
+
+            .pushsection __jump_table,  "aw"
+            .align 3
+            .long 1b - ., 3f - .
+            .quad {0} + {1} - .
+            .popsection
+
+            2: mov {2:w}, 0
+            3:
+            "#,
+            sym $key,
+            const ::core::mem::offset_of!($keytyp, $field),
+            inout(reg) output
+        );
+
+        output != 0
+    }};
+}
+
+#[doc(hidden)]
+#[macro_export]
+#[cfg(target_arch = "aarch64")]
+#[cfg(CONFIG_HAVE_RUST_ASM_GOTO)]
+macro_rules! _static_key_false {
+    ($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`.
+        const _: () = unsafe {
+            let key: *const $keytyp = ::core::mem::addr_of!($key);
+            let _: *const $crate::bindings::static_key = ::core::mem::addr_of!((*key).$field);
+        };
+
+        $crate::static_key::_static_key_false! { $($args)* }
+    }};
+}
+
+pub use {_static_key_false, static_key_false};

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