[PATCH v5 1/2] rust: add 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.

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.

One disadvantage of this patch is that it introduces a fair amount of
duplicated inline assembly. However, this is a limited and temporary
situation:

1. Most inline assembly has no reason to be duplicated like this. It is
   only needed here due to the use of 'i' parameters.

2. Alice will submit a patch along the lines of [1] that removes the
   duplication. This will happen as a follow-up to this patch series.

Comments are added to the C side as a reminder to change the asm on the
Rust side in case the C side is changed. For simplicity, the x86 asm
uses the forced 5 byte version. This will be fixed together with the
upcoming removal of the duplication.

Reviewed-by: Gary Guo <gary@garyguo.net>
Tested-by: WANG Rui <wangrui@loongson.cn>
Link: https://lore.kernel.org/rust-for-linux/20240801102804.GQ33588@noisy.programming.kicks-ass.net/ [1]
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 arch/arm64/include/asm/jump_label.h      |  1 +
 arch/loongarch/include/asm/jump_label.h  |  1 +
 arch/riscv/include/asm/jump_label.h      |  1 +
 arch/x86/include/asm/jump_label.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                       |  2 ++
 rust/kernel/static_key.rs                | 32 +++++++++++++++++++++++++++
 scripts/Makefile.build                   |  2 +-
 12 files changed, 205 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/jump_label.h b/arch/arm64/include/asm/jump_label.h
index 4e753908b801..56c106669f99 100644
--- a/arch/arm64/include/asm/jump_label.h
+++ b/arch/arm64/include/asm/jump_label.h
@@ -15,6 +15,7 @@
 
 #define JUMP_LABEL_NOP_SIZE		AARCH64_INSN_SIZE
 
+/* Changes to this asm must be reflected in `rust/kernel/arch/arm64/jump_label.rs` */
 #define JUMP_TABLE_ENTRY(key, label)			\
 	".pushsection	__jump_table, \"aw\"\n\t"	\
 	".align		3\n\t"				\
diff --git a/arch/loongarch/include/asm/jump_label.h b/arch/loongarch/include/asm/jump_label.h
index 29acfe3de3fa..3bdf0d834e8c 100644
--- a/arch/loongarch/include/asm/jump_label.h
+++ b/arch/loongarch/include/asm/jump_label.h
@@ -13,6 +13,7 @@
 
 #define JUMP_LABEL_NOP_SIZE	4
 
+/* Changes to this asm must be reflected in `rust/kernel/arch/loongarch/jump_label.rs` */
 #define JUMP_TABLE_ENTRY				\
 	 ".pushsection	__jump_table, \"aw\"	\n\t"	\
 	 ".align	3			\n\t"	\
diff --git a/arch/riscv/include/asm/jump_label.h b/arch/riscv/include/asm/jump_label.h
index 1c768d02bd0c..8ed63f588243 100644
--- a/arch/riscv/include/asm/jump_label.h
+++ b/arch/riscv/include/asm/jump_label.h
@@ -16,6 +16,7 @@
 
 #define JUMP_LABEL_NOP_SIZE 4
 
+/* Changes to this asm must be reflected in `rust/kernel/arch/riscv/jump_label.rs` */
 static __always_inline bool arch_static_branch(struct static_key * const key,
 					       const bool branch)
 {
diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
index cbbef32517f0..687fff131afe 100644
--- a/arch/x86/include/asm/jump_label.h
+++ b/arch/x86/include/asm/jump_label.h
@@ -12,6 +12,7 @@
 #include <linux/stringify.h>
 #include <linux/types.h>
 
+/* Changes to this asm must be reflected in `rust/kernel/arch/x86/jump_label.rs` */
 #define JUMP_TABLE_ENTRY				\
 	".pushsection __jump_table,  \"aw\" \n\t"	\
 	_ASM_ALIGN "\n\t"				\
diff --git a/rust/kernel/arch/arm64/jump_label.rs b/rust/kernel/arch/arm64/jump_label.rs
new file mode 100644
index 000000000000..5eede2245718
--- /dev/null
+++ b/rust/kernel/arch/arm64/jump_label.rs
@@ -0,0 +1,34 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024 Google LLC.
+
+//! Arm64 Rust implementation of jump_label.h
+
+/// arm64 implementation of arch_static_branch
+#[macro_export]
+#[cfg(target_arch = "aarch64")]
+macro_rules! arch_static_branch {
+    ($key:path, $keytyp:ty, $field:ident, $branch:expr) => {'my_label: {
+        core::arch::asm!(
+            r#"
+            1: nop
+
+            .pushsection __jump_table,  "aw"
+            .align 3
+            .long 1b - ., {0} - .
+            .quad {1} + {2} + {3} - .
+            .popsection
+            "#,
+            label {
+                break 'my_label true;
+            },
+            sym $key,
+            const ::core::mem::offset_of!($keytyp, $field),
+            const $crate::arch::bool_to_int($branch),
+        );
+
+        break 'my_label false;
+    }};
+}
+
+pub use arch_static_branch;
diff --git a/rust/kernel/arch/loongarch/jump_label.rs b/rust/kernel/arch/loongarch/jump_label.rs
new file mode 100644
index 000000000000..8d31318aeb11
--- /dev/null
+++ b/rust/kernel/arch/loongarch/jump_label.rs
@@ -0,0 +1,35 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024 Google LLC.
+
+//! Loongarch Rust implementation of jump_label.h
+
+/// loongarch implementation of arch_static_branch
+#[doc(hidden)]
+#[macro_export]
+#[cfg(target_arch = "loongarch64")]
+macro_rules! arch_static_branch {
+    ($key:path, $keytyp:ty, $field:ident, $branch:expr) => {'my_label: {
+        core::arch::asm!(
+            r#"
+            1: nop
+
+            .pushsection __jump_table,  "aw"
+            .align 3
+            .long 1b - ., {0} - .
+            .quad {1} + {2} + {3} - .
+            .popsection
+            "#,
+            label {
+                break 'my_label true;
+            },
+            sym $key,
+            const ::core::mem::offset_of!($keytyp, $field),
+            const $crate::arch::bool_to_int($branch),
+        );
+
+        break 'my_label false;
+    }};
+}
+
+pub use arch_static_branch;
diff --git a/rust/kernel/arch/mod.rs b/rust/kernel/arch/mod.rs
new file mode 100644
index 000000000000..14271d2530e9
--- /dev/null
+++ b/rust/kernel/arch/mod.rs
@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024 Google LLC.
+
+//! Architecture specific code.
+
+#[cfg_attr(target_arch = "aarch64", path = "arm64")]
+#[cfg_attr(target_arch = "x86_64", path = "x86")]
+#[cfg_attr(target_arch = "loongarch64", path = "loongarch")]
+#[cfg_attr(target_arch = "riscv64", path = "riscv")]
+mod inner {
+    pub mod jump_label;
+}
+
+pub use self::inner::*;
+
+/// A helper used by inline assembly to pass a boolean to as a `const` parameter.
+///
+/// Using this function instead of a cast lets you assert that the input is a boolean, rather than
+/// some other type that can be cast to an integer.
+#[doc(hidden)]
+pub const fn bool_to_int(b: bool) -> i32 {
+    b as i32
+}
diff --git a/rust/kernel/arch/riscv/jump_label.rs b/rust/kernel/arch/riscv/jump_label.rs
new file mode 100644
index 000000000000..2672e0c6f033
--- /dev/null
+++ b/rust/kernel/arch/riscv/jump_label.rs
@@ -0,0 +1,38 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024 Google LLC.
+
+//! RiscV Rust implementation of jump_label.h
+
+/// riscv implementation of arch_static_branch
+#[macro_export]
+#[cfg(target_arch = "riscv64")]
+macro_rules! arch_static_branch {
+    ($key:path, $keytyp:ty, $field:ident, $branch:expr) => {'my_label: {
+        core::arch::asm!(
+            r#"
+            .align  2
+            .option push
+            .option norelax
+            .option norvc
+            1: nop
+            .option pop
+            .pushsection __jump_table,  "aw"
+            .align 3
+            .long 1b - ., {0} - .
+            .dword {1} + {2} + {3} - .
+            .popsection
+            "#,
+            label {
+                break 'my_label true;
+            },
+            sym $key,
+            const ::core::mem::offset_of!($keytyp, $field),
+            const $crate::arch::bool_to_int($branch),
+        );
+
+        break 'my_label false;
+    }};
+}
+
+pub use arch_static_branch;
diff --git a/rust/kernel/arch/x86/jump_label.rs b/rust/kernel/arch/x86/jump_label.rs
new file mode 100644
index 000000000000..383bed273c50
--- /dev/null
+++ b/rust/kernel/arch/x86/jump_label.rs
@@ -0,0 +1,35 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024 Google LLC.
+
+//! X86 Rust implementation of jump_label.h
+
+/// x86 implementation of arch_static_branch
+#[macro_export]
+#[cfg(target_arch = "x86_64")]
+macro_rules! arch_static_branch {
+    ($key:path, $keytyp:ty, $field:ident, $branch:expr) => {'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} + {3} - .
+            .popsection
+            "#,
+            label {
+                break 'my_label true;
+            },
+            sym $key,
+            const ::core::mem::offset_of!($keytyp, $field),
+            const $crate::arch::bool_to_int($branch),
+        );
+
+        break 'my_label false;
+    }};
+}
+
+pub use arch_static_branch;
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 274bdc1b0a82..da1f69868d0d 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -27,6 +27,7 @@
 extern crate self as kernel;
 
 pub mod alloc;
+pub mod arch;
 #[cfg(CONFIG_BLOCK)]
 pub mod block;
 mod build_assert;
@@ -44,6 +45,7 @@
 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..32cf027ef091
--- /dev/null
+++ b/rust/kernel/static_key.rs
@@ -0,0 +1,32 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024 Google LLC.
+
+//! Logic for static keys.
+
+use crate::bindings::*;
+
+/// 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 {
+    ($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::arch::jump_label::arch_static_branch! { $key, $keytyp, $field, false }
+    }};
+}
+
+pub use 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

-- 
2.46.0.rc2.264.g509ed76dc8-goog