linux-kernel - Re: [PATCH] perf_events: AMD event scheduling (v3)

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Date:	Wed, 10 Feb 2010 14:28:21 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, paulus@...ba.org,
	davem@...emloft.net, fweisbec@...il.com, robert.richter@....com,
	perfmon2-devel@...ts.sf.net, eranian@...il.com
Subject: Re: [PATCH] perf_events: AMD event scheduling (v3)

On Wed, 2010-02-10 at 14:17 +0100, Peter Zijlstra wrote:
> On Wed, 2010-02-10 at 14:04 +0100, Stephane Eranian wrote:
> 
> > > @@ -2268,7 +2268,7 @@ static inline int amd_is_nb_event(struct
> > >        u64 val = hwc->config & K7_EVNTSEL_EVENT_MASK;
> > >        /* event code : bits [35-32] | [7-0] */
> > >        val = (val >> 24) | (val & 0xff);
> > > -       return val >= 0x0e0;
> > > +       return val >= 0xe00;
> > >  }
> > >
> > I don't understand the change from 0xe0 to 0xe00.
> > That's not the same thing at all.
> > Event select is bits 0-7 + 32-35.
> 
> OK that appears to be my bad, because you extended K7_EVNTSEL_EVENT_MASK
> with bit 35 I thought NB events all had bit 35 set.
> 
> But looking at the AMD docs it does indeed appear to start at 0xe0, and
> there are no events with bit 35 set, only a few with bit 32.
> 
> I'll switch it back to 0xe0.

Fwiw, for the purpose of that function you might as well write:

static inline int amd_is_nb_event(struct hw_perf_event *hwc)
{
	return (hwc->config & K7_EVNTSEL_EVENT_MASK) > 0xe0;
}

No need to move bits around higher than the value you compare against.

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