linux-kernel - Re: [PATCH] perf_events: fix validate

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Message-ID: <19212.22371.633313.129961@cargo.ozlabs.ibm.com>
Date:	Wed, 25 Nov 2009 09:00:03 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	eranian@...il.com
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, mingo@...e.hu,
	perfmon2-devel@...ts.sf.net, eranian@...gle.com
Subject: Re: [PATCH] perf_events: fix validate_event bug

stephane eranian writes:

> That means we can drop is_software_event() in this code and instead
> define locally
> in x86 a is_hw_pmu_event() function as event->pmu == &pmu.

I'd have to see the patch, but that doesn't feel entirely right,
because there is a unique characteristic of software events, compared
to hardware or breakpoint events: they are never capacity
constrained.  In the past, only hardware events were capacity
constrained, which meant that all the decisions about whether a group
could go on could be done in the hardware PMU backend.  Now we have
two sources of capacity constraints, so it may be that a group can't
go on even if the hardware PMU has capacity.  That's going to be
somewhat interesting to get completely right, I think.

Paul.
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