linux-kernel - Re: re-enable Nehalem raw Offcore-Events support

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Message-ID: <BANLkTimjmYUJMGw0aB+w+E76H+kxSpqFZg@mail.gmail.com>
Date:	Fri, 29 Apr 2011 20:17:24 +0300
From:	Pekka Enberg <penberg@...nel.org>
To:	Vince Weaver <vweaver1@...s.utk.edu>
Cc:	torvalds@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Stephane Eranian <eranian@...il.com>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: re-enable Nehalem raw Offcore-Events support

Hi Vince,

On Fri, Apr 29, 2011 at 6:04 PM, Vince Weaver <vweaver1@...s.utk.edu> wrote:
> Hello Linus
>
> can you revert the commit b52c55c6a25e4515b5e075a989ff346fc251ed09
>
> This removed functionality from perf_events that allowed raw event access
> for OFFCORE_EVENTS type events on Nehalem and Westmere cpus.
>
> To be fair, this is not technically a regression as the feature was only
> (finally!) added in the 2.6.39 merge window.  However this is a useful
> feature and many tools (including the PAPI performance counter library
> that I work on) had added support for it in anticipation of the 2.6.39
> release.
>
> Ingo's reasons for removing the feature seem to boil down to
>  1.  "perf" doesn't use the functionality, and any other userspace
>      program that uses the perf_events syscalls don't matter
>  2.  Users are too stupid to use the raw functionality properly;
>      we should only allow a kernel-developer-approved small subset
>      of the features provided by the CPU as described in the intel
>      developers manuals.
>
> #2 seems like a gross misinterpretation of the whole "Linux gives you
> enough rope to shoot yourself in the foot" policy from days passed, but
> maybe things have moved on.

That's a gross misrepresentation of what Ingo has been saying on LKML.
Really, learn to work with relevant maintainers before you ask Linus
to revert something.

                                Pekka
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