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Date: Tue, 6 Feb 2018 11:54:45 +0100
From: Ingo Molnar <mingo@kernel.org>
To: David Woodhouse <dwmw2@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>,
        linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        David Ahern <dsahern@gmail.com>, Fenghua Yu <fenghua.yu@intel.com>,
        Jiri Olsa <jolsa@kernel.org>, Namhyung Kim <namhyung@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>, Wang Nan <wangnan0@huawei.com>
Subject: Re: [PATCH 05/10] tools headers: Synchoronize x86 features UAPI
 headers
Message-ID: <20180206105445.x3gqpaa3rwbnhugo@gmail.com>
References: <20180205195620.9010-1-acme@kernel.org>
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* David Woodhouse <dwmw2@infradead.org> wrote:

> 
> 
> On Mon, 2018-02-05 at 16:56 -0300, Arnaldo Carvalho de Melo wrote:
> > 
> > None will entail changes in the tools/perf/, synchronizing to elliminate
> > these perf build warnings:
> > 
> >   Warning: Kernel ABI header at 'tools/arch/x86/include/asm/disabled-features.h' differs from latest version at 'arch/x86/include/asm/disabled-features.h'
> >   Warning: Kernel ABI header at 'tools/arch/x86/include/asm/required-features.h' differs from latest version at 'arch/x86/include/asm/required-features.h'
> >   Warning: Kernel ABI header at 'tools/arch/x86/include/asm/cpufeatures.h' differs from latest version at 'arch/x86/include/asm/cpufeatures.h'
> 
> Ick. Have we considered just using a symlink? Why have copies of the
> same header file in different places in the tree, and tooling to
> complain i̶f̶when they get out of sync?

In the past we had incidents where kernel changes broke tooling and vice versa.

This is basically a soft-COW mechanism that decouples tooling source code from the 
kernel source code, while still having a technological mechanism in place that 
encourages the syncing of header files.

Thanks,

	Ingo