Alice Ryhl of Google has been working on an improvement to the Linux kernel code for inlining C helpers into Rust when making use of a Link-Time Optimized (LTO) kernel build. At least some of the patches are queued up for merging in the upcoming Linux 6.20~7.0 cycle for helping those enabling the Rust kernel support and also making use of the LLVM/Clang compiler’s LTO capabilities for greater performance.

This work to inline C helpers into Rust when using LTO involves adding a "__rust_helper" annotation to every Rust helper within the kernel. This "__rust_helper" is needed since C helpers cannot be inlined into Rust when using LTO because LLVM detects slightly different options on the code generation units.

In turn when using LTO for the kernel builds, "__always_inline" will be added to the helpers except when running bindgen to avoid an issue there with ignoring functions markd as inline. There is a 46 patch series for applying this "__rust_helper" where needed throughout the kernel code. Those patches don’t need to be merged in one-go and thus left up to the individual subsystem maintainers for taking the patches but at least some of them are set for merging with Linux 7.0.

Via the vfs-7.0.rust Git branch of VFS.git, the patches there have been taken for inlining the C helpers into Rust. We’ll see if the full patch series via the different branches all get accepted for this next kernel cycle or will take longer to upstream it all. A separate patch will then take to wiring up the "__rust_helper" behavior.

Separately, for those wondering about the performance benefits of using Link Time Optimizations when building the Linux kernel with LLVM/Clang, recently I ran some fresh benchmarks within Some Meaningful Performance Benefits For Clang + LTO Built Linux Kernels.