linux-kernel - Re: [RFC 0/5] CR4 handling improvements

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Message-ID: <alpine.DEB.2.10.1410210003300.5251@pianoman.cluster.toy>
Date:	Tue, 21 Oct 2014 00:06:54 -0400 (EDT)
From:	Vince Weaver <vince@...ter.net>
To:	Andy Lutomirski <luto@...capital.net>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Kees Cook <keescook@...omium.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Erik Bosman <ebn310@....vu.nl>
Subject: Re: [RFC 0/5] CR4 handling improvements

On Tue, 14 Oct 2014, Andy Lutomirski wrote:

> This little series tightens up rdpmc permissions.  With it applied,
> rdpmc can only be used if a perf_event is actually mmapped.  For now,
> this is only really useful for seccomp.

So just to be difficult...

I am aware of at least one group who is doing low-latency performance
measures using rdpmc on Linux.

They start the counters manually by poking the MSRs directly (bypassing 
perf_event_open()).

They use rdpmc, grateful for the fact that currently CR4 is set up so they 
can do this w/o patching the kernel.

These patches of course would break this use case...

Vince
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