linux-kernel - Re: [PATCH v2 09/28] kbuild: add support for Clang LTO

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Message-ID: <20200908171052.GB2743468@google.com>
Date:   Tue, 8 Sep 2020 10:10:52 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-pci@...r.kernel.org, X86 ML <x86@...nel.org>
Subject: Re: [PATCH v2 09/28] kbuild: add support for Clang LTO

On Sun, Sep 06, 2020 at 04:36:32AM +0900, Masahiro Yamada wrote:
> On Fri, Sep 4, 2020 at 5:31 AM Sami Tolvanen <samitolvanen@...gle.com> wrote:
> >
> > This change adds build system support for Clang's Link Time
> > Optimization (LTO). With -flto, instead of ELF object files, Clang
> > produces LLVM bitcode, which is compiled into native code at link
> > time, allowing the final binary to be optimized globally. For more
> > details, see:
> >
> >   https://llvm.org/docs/LinkTimeOptimization.html
> >
> > The Kconfig option CONFIG_LTO_CLANG is implemented as a choice,
> > which defaults to LTO being disabled.
> 
> What is the reason for doing this in a choice?
> To turn off LTO_CLANG for compile-testing?
> 
> I would rather want to give LTO_CLANG more chances
> to be enabled/tested.

It's a choice to prevent LTO from being enabled by default with
allyesconfig and allmodconfig. It would take hours to build these even on a
fast computer, and probably days on older hardware.

> > +ifdef CONFIG_LTO_CLANG
> > +ifdef CONFIG_THINLTO
> > +CC_FLAGS_LTO_CLANG := -flto=thin -fsplit-lto-unit
> > +KBUILD_LDFLAGS += --thinlto-cache-dir=.thinlto-cache
> 
> 
> I think this would break external module builds
> because it would create cache files in the
> kernel source tree.
> 
> External module builds should never ever touch
> the kernel tree, which is usually located under
> the read-only /usr/src/ in distros.
> 
> 
> .thinlto-cache should be created in the module tree
> when it is built with M=.

Thanks for pointing this out, I'll fix the path in v3.

> >  # Directories & files removed with 'make distclean'
> > -DISTCLEAN_FILES += tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS
> > +DISTCLEAN_FILES += tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS .thinlto-cache
> 
> 
> 
> This was suggested in v1, but I could not understand
> why doing this in distclean was appropriate.
> 
> Is keeping cache files of kernel objects
> useful for external module builds?

No, the cache only speeds up incremental kernel builds.

> Also, please clean up .thinlto-cache for external module builds.

Ack.

Sami