kpatch-build: Enable cross compiling with env TARGET_ARCH

[liu-song-6]

llvm/clang supports cross compiling of the kernel. To build livepatch in the same cross compile environment, enable specifying TARGET_ARCH for kpatch-build. For example, in a x86_64 host, we can build livepatch for aarch64 kernel with:

TARGET=aarch64 kpatch-build ...

[liu-song-6]

I am creating this PR for comments. We probably want to wait until #1439 is landed.

[joe-lawrence]

This is interesting and something I hoped we'd eventually implement :) Have you run cross-compiled kpatch modules against native-built kernels?

[liu-song-6]

Have you run cross-compiled kpatch modules against native-built kernels?

I haven't tried this, but I think this should work. The compiler should create the same binary in native compile and cross compile.

In practice, I assume we will use identical environment to build the kernel and the kpatch modules.

[liu-song-6]

With the aarch64 support landed, can we merge this PR and cut a new release?

Thanks!

[joe-lawrence]

Hi @liu-song-6 : we should probably document usage and any caveats for this. For example, is this clang/llvm only or what happens if used with gcc? Can it be used to kpatch native-built binaries as well? (I'm not sure if this belongs as a FAQ or short blurb on the README.md or developer's guide, etc?)

[liu-song-6]

Hi @liu-song-6 : we should probably document usage and any caveats for this. For example, is this clang/llvm only or what happens if used with gcc? Can it be used to kpatch native-built binaries as well? (I'm not sure if this belongs as a FAQ or short blurb on the README.md or developer's guide, etc?)

[joe-lawrence]

Thanks @liu-song-6 , I dropped out some of the extraneous whitespace edits (probably an editor setting).

I'll work out a release next week (need to run all the internal tests and then push out a tag).