linux-kernel - Re: [patch 2/3] performance counters: documentation

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Message-Id: <20081204.192650.193940655.davem@davemloft.net>
Date:	Thu, 04 Dec 2008 19:26:50 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	arjan@...radead.org
Cc:	paulus@...ba.org, tglx@...utronix.de, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, akpm@...ux-foundation.org,
	mingo@...e.hu, eranian@...glemail.com, dada1@...mosbay.com,
	robert.richter@....com, hpa@...or.com, a.p.zijlstra@...llo.nl,
	rostedt@...dmis.org
Subject: Re: [patch 2/3] performance counters: documentation

From: Arjan van de Ven <arjan@...radead.org>
Date: Thu, 4 Dec 2008 18:50:02 -0800

> I would like to respectfully disagree with this some. The kernel needs
> to abstract hardware to some degree for userspace. The problem in this
> case is that userspace can't really do a better job, in fact it can
> only do a worse job since it lacks the coordination capability of
> knowing it has full control of all the hardware registers. 

The perfmon context abstraction dealt with that.  Code using the
perfmon interfaces provided a set of counter and control register
values to the kernel.

The kernel merely loaded and unloaded them when a process (or group of
processes) ran.

The kernel is a validity checker, and that minimal stuff is exactly
what the perfmon kernel component implemented.
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