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

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Message-ID: <202011211204.211E2B12@keescook>
Date:   Sat, 21 Nov 2020 12:11:25 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Will Deacon <will@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-pci@...r.kernel.org
Subject: Re: [PATCH v7 02/17] kbuild: add support for Clang LTO

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