linux-kernel - Re: [PATCH 0/4] perf: Intel uncore pmu counting support

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Date:	Fri, 1 Jul 2011 14:28:20 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Lin Ming <ming.m.lin@...el.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/4] perf: Intel uncore pmu counting support

On Fri, Jul 1, 2011 at 2:23 PM, Stephane Eranian <eranian@...gle.com> wrote:
> Lin,
>
> Ok, false alarm. Vol3b is correct. I missed the fact that if you want
> wrmsrl() to write full width generic counters, you have to use the
> alternate MSR alias. I guess that's for backward compatibility.
> So for full width wrmsrl on PERFCTR0 -> use 0x41c
>
> uncore counter value: 0x678ffffeeee
> core fixed counter value: 0x678ffffeeee
>       core generic counter value: 0xffffffffeeee
> wide core generic counter value: 0x678ffffeeee
>
> I think full width write to fixed counters has been there
> for a long time. Looks like the problem was only present
> for core generic counters until SNB.
>
Forgot to say, that we could adapt the code to use the alternate
registers if full width wrmsrl() is supported. That would mitigate
a bit the overhead of sampling and counting by simply not interrupting
that often to accumulate counts in the SW counter.
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