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Date:   Thu, 9 Nov 2017 07:48:06 +0300
From:   Maxim Kuvyrkov <maxim.kuvyrkov@...aro.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Yury Norov <ynorov@...iumnetworks.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Greg Hackmann <ghackmann@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Alex Matveev <alxmtvv@...il.com>,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 13/15] arm64: fix mrs_s/msr_s macros for clang LTO

> On Nov 9, 2017, at 3:02 AM, Andi Kleen <ak@...ux.intel.com> wrote:
> 
>> There's the series from Andi Kleen that enables LTO for Linux on x86:
>> https://lwn.net/Articles/512548/
>> https://github.com/andikleen/linux-misc/tree/lto-411-1
>> 
>> It has solved many problems you also try to solve, and some patches
>> are looking very similar. 
>> 
>> At now we have different patchsets for gcc and clang, and it would be
>> better to have them together. One thing I'm worried is that you introduce
>> CONFIG_CLANG_LTO and use it for all cases, including that where more
>> generic CONFIG_LTO should be used.
> 
> Yes would be good to merge the two. I've been looking at updating
> my old one.
> 
> I don't cover any ARM code, but lots of generic code. My patches
> also worked on MIPS at least.
> 
> There's also older patches to enable single-pass-linking for kallsyms,
> which is extremly useful for LTO build performance.

[Yury, thanks for the CC:]

Chiming in from the toolchain side, Linaro's Toolchain team will try to help with any GCC or Clang issues that are exposed by building kernel with LTO on arm64 / arm.

Regarding CONFIG_* options, I would expect most of the configuration changes to be equally valid for both GCC's and Clang's LTO support.  Sami, I don't think it's fair to ask you to support both Clang and GCC in your patchset, but, where changes are obviously toolchain-agnostic, could you use CONFIG_LTO?  And use CONFIG_LTO_CLANG for Clang-specific parts?

This way we will be able to avoid most of the refactoring when adding support for GCC's LTO.

Thank you,

--
Maxim Kuvyrkov
www.linaro.org

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