Re: [PATCH v7 02/17] kbuild: add support for Clang LTO

[Kees Cook <keescook@chromium.org>]

On Fri, Nov 20, 2020 at 05:46:44PM -0800, Sami Tolvanen wrote:
> Sure, this looks good to me, I'll use this in v8. The only minor
> concern I have is that ThinLTO cannot be set as the default LTO mode,
> but I assume anyone who selects LTO is also capable of deciding which
> mode is better for them.

It could be re-arranged similar to what you had before, but like:

config LTO
	bool "..."
	depends on HAS_LTO
	help
	  ...

choice
	prompt "LTO mode" if LTO
	default LTO_GCC if HAS_LTO_GCC
	default LTO_CLANG_THIN if HAS_LTO_CLANG
	default LTO_CLANG_FULL
	help
	  ...

	config LTO_CLANG_THIN
	...

	config LTO_CLANG_FULL
endchoice

Then the LTO is top-level yes/no, but depends on detected capabilities,
and the mode is visible if LTO is chosen, etc.

I'm not really sure which is better...

> > +config LTO_CLANG_THIN
> > +       bool "Clang ThinLTO (EXPERIMENTAL)"
> > +       depends on ARCH_SUPPORTS_LTO_CLANG_THIN
> > +       select LTO_CLANG
> > +       help
> > +         This option enables Clang's ThinLTO, which allows for parallel
> > +         optimization and faster incremental compiles compared to the
> > +         CONFIG_LTO_CLANG_FULL option. More information can be found
> > +         from Clang's documentation:
> > +
> > +           https://clang.llvm.org/docs/ThinLTO.html
> > +
> > +         If unsure, say Y.
> >  endchoice
> 
> The two LTO_CLANG_* options need to depend on HAS_LTO_CLANG, of course.

Whoops, yes. Thanks for catching that. :)

-- 
Kees Cook