linux-kernel - Re: II.1 - Fixed counters on Intel

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Message-ID: <20090622115703.GM24366@elte.hu>
Date:	Mon, 22 Jun 2009 13:57:03 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	eranian@...il.com
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Robert Richter <robert.richter@....com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Andi Kleen <andi@...stfloor.org>,
	Maynard Johnson <mpjohn@...ibm.com>,
	Carl Love <cel@...ibm.com>,
	Corey J Ashford <cjashfor@...ibm.com>,
	Philip Mucci <mucci@...s.utk.edu>,
	Dan Terpstra <terpstra@...s.utk.edu>,
	perfmon2-devel <perfmon2-devel@...ts.sourceforge.net>
Subject: Re: II.1 - Fixed counters on Intel

> II/ X86 comments
>
>  1/ Fixed counters on Intel
>
> You cannot simply fall back to generic counters if you cannot find
> a fixed counter. There are model-specific bugs, for instance
> UNHALTED_REFERENCE_CYCLES (0x013c), does not measure the same
> thing on Nehalem when it is used in fixed counter 2 or a generic
> counter. The same is true on Core.

This could be handled via a model specific quirk, if the erratum is
serious enough.

> You cannot simply look at the event field code to determine
> whether this is an event supported by a fixed counters. You must
> look at the other fields such as edge, invert, cnt-mask. If those
> are present then you have to fall back to using a generic counter
> as fixed counters only support priv level filtering. As indicated
> above, though, programming UNHALTED_REFERENCE_CYCLES on a generic
> counter does not count the same thing, therefore you need to fail
> if filters other than priv levels are present on this event.

Agreed, we'll fix this.
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