Re: [PATCH v4 1/2] rust: add static_key_false

On Wed, Jul 31, 2024 at 7:05 PM Peter Zijlstra [off-list ref] wrote:

On Fri, Jun 28, 2024 at 01:23:31PM +0000, Alice Ryhl wrote:

quoted

 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 +-
 8 files changed, 201 insertions(+), 1 deletion(-)

So I really find the amount of duplicated asm offensive. Is is far too
easy for any of this to get out of sync.

quoted

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;
+    }};
+}

Note that this uses the forced 5 byte version, and not the dynamic sized
one. On top of that it hard-codes the nop5 string :/

Please work harder to not have to duplicate stuff like this.

I really didn't want to duplicate it, but it's very hard to find a
performant alternative. Is there any way we could accept duplication
only in the cases where an 'i' parameter is used? I don't have the
choice of using a Rust helper for 'i' parameters.

Perhaps one option could be to put the Rust code inside jump_label.h
and have the header file evaluate to either C or Rust depending on the
value of some #ifdefs?

#ifndef RUST_ASM
/* existing C code goes here */
#endif
#ifdef RUST_ASM
// rust code goes here
#endif

That way the duplication is all in a single file. It would also avoid
the need for duplicating the nop5 string, as the Rust case is still
going through the C preprocessor and can use the existing #define.

I'm also open to other alternatives. But I don't have infinite
resources to drive major language changes.

Alice