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diff --git a/content/16/index.md b/content/16/index.md new file mode 100644 index 0000000..2cb12b3 --- /dev/null +++ b/content/16/index.md @@ -0,0 +1,42 @@ ++++ +date = 2022-02-05T19:29:44+01:00 +title = "Building the Linux kernel with clang and full LTO" + +[taxonomies] +tags = ["experiments"] ++++ + +My main desktop PC tracks the latest LTS [release](https://kernel.org/) of the +Linux kernel which very recently switched to the 5.15 line. Along with neat new +features like the +[NTSF3 driver](https://lore.kernel.org/all/aa4aa155-b9b2-9099-b7a2-349d8d9d8fbd@paragon-software.com/) +it also includes experimental support for +[Link Time Optimization](https://llvm.org/docs/LinkTimeOptimization.html) +through LLVM's +[`clang` compiler](https://clang.llvm.org/). + +I'm not really one to shy away from weird experiments, so I decided to run a +full LTO kernel for a while. If you have a recent version of `clang` and the +[`lld` linker](https://lld.llvm.org/), building one is as easy as toggling +`CONFIG_LTO_CLANG_FULL` and exporting the right flags to `make`: + + make CC=clang LLVM=1 menuconfig + # CONFIG_LTO_CLANG_FULL=y + make CC=clang LLVM=1 + +Subsequent steps are the same as with a normal build: + + sudo make install + sudo make modules_install + +Keep in mind, however, that any out-of-tree modules such as ZFS must also be +built with `clang`. Here I ran into +[this bug](https://bugs.gentoo.org/show_bug.cgi?id=814194) which should soon be +[fixed upstream](https://github.com/openzfs/zfs/pull/13046). For now I +backported that fix locally to ZFS 2.1.2 and am building it like so: + + sudo CC=clang LLVM=1 emerge zfs-kmod + +Build times and memory usage when building are increased dramatically with full +LTO. Optimizing `vmlinux.o` alone allocates about 3 to 4 GiB of memory. If you +rely a lot on incremental builds, thin LTO might be the better option here. |