On Wed, Mar 10, 2021 at 10:47 PM Nicolas Pitre [off-list ref] wrote:
...

quoted

With CONFIG_TRIM_UNUSED_KSYMS=y I see a 3x-loops of building .version
and folowing steps - got no answer if this is intended.

Yes it is intended. I explained it here:

https://lkml.org/lkml/2021/3/9/1099

Ah, cool.
Thanks for that link.

With CONFIG_TRIM_UNUSED_KSYMS some EXPORT_SYMBOL() are removed, which
allows for optimizing away the corresponding code, which in turn opens
the possibility for more EXPORT_SYMBOL() to be removed, etc. The process
eventually converge to a stable build. Normally only 2 passes are needed
to converge, but LTO opens the possibilities for extra passes.

quoted

Means longer build-time.

Oh, absolutely.  LTO (at least when I played with it) is slow. Add the
multi-pass from CONFIG_TRIM_UNUSED_KSYMS on top of that and your kernel
build becomes agonizingly slow. This is not something you want when
doing kernel development.

Thanks for the feedback.

quoted

I did not follow this anymore as both Kconfigs with Clang-LTO consume
more build-time and the resulting vmlinux is some MiB bigger than with
Clang-CFI.

That's rather strange. At least with gcc LTO I always obtained smaller
kernels.

I cannot say much to GCC-LTO - I never used it.

If you are interested in Clang-CFI (see [1]) - which requires
Clang-LTO enabled and LLVM/Clang >= 12.
Some hours ago version 12.0.0-rc3 was released, see [2].

- Sedat -

[1] https://github.com/samitolvanen/linux/commits/clang-cfi
[2] https://github.com/ClangBuiltLinux/linux/issues/1259

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