linux-kernel - Re: [PATCH v9] pgo: add clang's Profile Guided Optimization infrastructure

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Date:   Mon, 14 Jun 2021 09:22:58 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Marco Elver <elver@...gle.com>, Bill Wendling <morbo@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Fangrui Song <maskray@...gle.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>, johannes.berg@...el.com,
        oberpar@...ux.vnet.ibm.com, linux-toolchains@...r.kernel.org
Subject: Re: [PATCH v9] pgo: add clang's Profile Guided Optimization
 infrastructure

On Mon, Jun 14, 2021 at 05:35:45PM +0200, Peter Zijlstra wrote:
> On Mon, Jun 14, 2021 at 08:26:01AM -0700, Kees Cook wrote:
> > So, AFAICT, the original blocking issue ("PGO does not respect noinstr")
> > is not actually an issue (noinstr contains notrace, which IS respected
> > by PGO[2]), I think this is fine to move forward.
> 
> It is *NOT*: https://godbolt.org/z/9c7xdvGd9
> 
> Look at how both compilers generate instrumentation in the no_instr()
> function.

Well that's disappointing. I'll put this on hold until Clang can grow an
appropriate attribute (or similar work-around). Thanks for catching
that.

-- 
Kees Cook