linux-kernel - Re: [PATCH 04/29] x86/perfcounters: rework pmc_amd_save_disable_all() and pmc_amd_restore

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Message-ID: <20090429111159.GJ2373@elte.hu>
Date:	Wed, 29 Apr 2009 13:11:59 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Robert Richter <robert.richter@....com>,
	Paul Mackerras <paulus@...ba.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 04/29] x86/perfcounters: rework
	pmc_amd_save_disable_all() and pmc_amd_restore_all()


* Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> On Wed, 2009-04-29 at 12:47 +0200, Robert Richter wrote:
> > MSR reads and writes are expensive. This patch adds checks to avoid
> > its usage where possible.
> 
> save_disable_all()
> enable(1)
> restore_all()
> 
> would not correctly enable 1 with the below modification as we do 
> not write the configuration into the msr, on which restore relies, 
> as it only toggles the _ENABLE bit.
> 
> That said, I'm not sure if that's really an issue, but its why the 
> does does as it does.
> 
> A better abstraction could perhaps avoid this issue all-together.

Could we remove the disable-all facility altogether and make the 
core code NMI-safe? The current approach wont scale on CPUs that 
dont have global-disable features.

disable-all was arguably a hack i introduced and which spread too 
far. Can you see a conceptual need for it?

	Ingo
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