Support building with Clang

[Andrey Konovalov]

I see that README already mentions this, but I have some additional points on this, so I'll just put them here.

Kernel build allows to specify the compiler and the assembler separately. Ideally there's support for at least this three modes: compiler=GCC, assembler=GCC (the one that is implemented right now); compiler=Clang, assembler=GCC (the one I currently use with some hacks on top of the existing code); compiler=Clang, assembler=Clang (the one that eventually might also be useful; requires LLVM_IAS=1 provided to make).

There are also multiple different versions of GCC and Clang, which might need to be used in different combinations. E.g. initially I used Clang 10 + GCC 10, but recently switched to Clang 11 + GCC 10, as the kernel doesn't support Clang 10 anymore. Maybe this means that we need a single container with multiple different compilers installed and a way to choose between them.

[Alexander Popov]

Cool!
Yes, adding Clang support in kernel-build-containers is very useful.

What is your workflow in details?
Do you use Clang provided by the distro (Ubuntu in our case)?

I haven't tried to build the kernel with Clang yet.
So I would really appreciate your links to the tutorials / documentation about combining Clang and GCC for building the Linux kernel.

Thanks a lot!

[Andrey Konovalov]

Yes, adding Clang support in kernel-build-containers is very useful.

Yes, that would be quite cool! But even without Clang, this tool is awesome, thank you for it!

Do you use Clang provided by the distro (Ubuntu in our case)?

Yes, I modified the Dockerfile to use Ubuntu 20.10 and install clang-11 (or, previously, clang-10, which worked with Ubuntu 20.04).

What is your workflow in details?

Currently I use kernel-build-containers to cross-compile arm64 kernel with a bunch of different configs. I use Clang 11 as compiler and GCC 10 as assembler.

The quick hack I did to enable Clang (besides installing it) is:

--- a/make_linux.py
+++ b/make_linux.py
@@ -19,7 +19,8 @@ def get_cross_compile_args(arch):
     if arch == 'i386':
         return ['ARCH=i386']
     if arch == 'aarch64':
-        return ['ARCH=arm64', 'CROSS_COMPILE=aarch64-linux-gnu-']
+        return ['ARCH=arm64', 'CROSS_COMPILE=aarch64-linux-gnu-', 'CC=clang-11']

I've initially wanted to use Clang as assembler as well, but unfortunately arm64 kernel doesn't yet seem to assemble with Clang successfully.

[Alexander Popov]

Hi @xairy ! Hello everyone!

I've added the basic clang support in kernel-build-containers.

Now we have additional kernel-build-container:clang-12, which goes with clang-12 and gcc-10.

For the Linux kernel compilation, the make_linux.py utility automatically adds the proper CC argument depending on the chosen compiler. No need to worry about it. And the LLVM argument may be added manually after the -- delimiter.

The kernel build example:

$ python3 make_linux.py -a aarch64 -k ~/linux/experiment.config -s ~/linux/linux -o ~/linux/build_out -c clang-12 -- -j5
[+] Going to build the Linux kernel for aarch64
[+] Using "/home/a13x/linux/experiment.config" as kernel config
[+] Using "/home/a13x/linux/linux" as Linux kernel sources directory
[+] Using "/home/a13x/linux/build_out" as build output directory
[+] Going to build with: clang-12
[+] Have additional arguments for 'make': -j5

=== Building with clang-12 ===
Output subdirectory for this build: /home/a13x/linux/build_out/experiment__aarch64__clang-12
Output subdirectory already exists, use it (no cleaning!)
Copy kconfig to output subdirectory as ".config"
Going to save build log to "build_log.txt" in output subdirectory
Compiling with clang requires 'CC=clang'
Add arguments for cross-compilation: ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu-
Run the container: bash ./start_container.sh clang-12 /home/a13x/linux/linux /home/a13x/linux/build_out/experiment__aarch64__clang-12 -n make O=~/out/ CC=clang ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- -j5 2>&1
    Hey, we gonna use sudo for running docker
    Starting "kernel-build-container:clang-12"
    Source code directory "/home/a13x/linux/linux" is mounted at "~/src"
    Build output directory "/home/a13x/linux/build_out/experiment__aarch64__clang-12" is mounted at "~/out"
    Run docker in NON-interactive mode
    Gonna run "make O=~/out/ CC=clang ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- -j5 2>&1"
    
    make[1]: Entering directory '/home/a13x/out'
      SYNC    include/config/auto.conf.cmd
      GEN     Makefile
...
    make[1]: Leaving directory '/home/a13x/out'
The container returned 0
See build log: /home/a13x/linux/build_out/experiment__aarch64__clang-12/build_log.txt
Only remove the container id file:
    Hey, we gonna use sudo for running docker
    Search "container.id" file in build output directory "/home/a13x/linux/build_out/experiment__aarch64__clang-12"
    OK, "container.id" file exists, removing it
    OK, container 48a25a340a1ceb3d1ee4baa4eafb1b44ad98c6a70bd105f0376cffb2ba21bd2e doesn't run
Finished with the container

[+] Done, see the results

Commits providing this feature:
5013bc8
9f48f71
1b8695f
748e1e7
68a6801
60b8d5e