linux-kernel - Re: [PATCH] perf/x86/intel: Mark expected switch fall-throughs

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Date:   Tue, 25 Jun 2019 13:37:31 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Joe Perches <joe@...ches.com>, Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Kan Liang <kan.liang@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Shawn Landden <shawn@....icu>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Nathan Chancellor <natechancellor@...il.com>
Subject: Re: [PATCH] perf/x86/intel: Mark expected switch fall-throughs

On Tue, Jun 25, 2019 at 1:27 PM Nathan Chancellor
<natechancellor@...il.com> wrote:
> On Tue, Jun 25, 2019 at 09:53:09PM +0200, Thomas Gleixner wrote:
> > On Tue, 25 Jun 2019, Nathan Chancellor wrote:
> > > On Tue, Jun 25, 2019 at 10:12:42AM -0700, Kees Cook wrote:
> > > > On Tue, Jun 25, 2019 at 09:18:46AM +0200, Peter Zijlstra wrote:
> > > > > Can it build a kernel without patches yet? That is, why should I care
> > > > > what LLVM does?
> > > >
> > > > Yes. LLVM trunk builds and boots x86 now. As for distro availability,
> > > > AIUI, the asm-goto feature missed the 9.0 LLVM branch point, so it'll
> > > > appear in the following release.

Kees meant asm goto missed 8.0 LLVM branch.  9.0 which is unreleased
will have it.

> > Kernel boots. As I'm currently benchmarking VDSO performance, this was
> > obviosly my first test. Compared to the same kernel built with gcc6.3 the
> > performance of the VDSO drops slightly.
> >
> > It's below 1%. Though I need to run the same tests on 4 other uarchs to get
> > the full picture. This stuff is randomly changing behaviour accross uarchs
> > depending on how the c source is arranged. So nothing to worry about (yet).

Thank you very much for testing and for these reports.  We've been
working through:
1. make it build
2. make it boot
3. make it run well
4. add features

With some amount of cycles in the above.  Now that most of the build
issues have been resolved, we're more able to focus our resources on 3
and 4.  Please report unexpected regressions to our mailing list
clang-built-linux@...glegroups.com and we'll track/follow up upstream
on the LLVM side.
-- 
Thanks,
~Nick Desaulniers