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Subject: Re: [PATCH 5/9] perf: Adding sysfs group format attribute for pmu
 device
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Corey Ashford <cjashfor@linux.vnet.ibm.com>
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Date: Wed, 01 Feb 2012 14:36:24 +0100
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On Tue, 2012-01-31 at 17:25 -0800, Corey Ashford wrote:
> 
> One other comment that occurs to me is that it would be nice for perf to
> know when a supplied value is out of range, or will have undefined
> results.  For example, a field may be 8 bits wide, but not all 8-bit
> values are legal.  For example, there may be 208 events, and the codes
> may be somehwhat or even very sparsely encoded.  So, ideally, a config
> field in sysfs might look like this:
> 
> config1:0-7:0x0-0xd8,0xdb-0xe2,0xe4-0xe6
> 
> This way perf could check for valid values before stuffing them into
> registers, and give a good error message to the user.  If there is no
> restriction field, it would be assumed all of the possible values are valid.
> 
> I think the kernel code needs to check for bad values as well, because
> people can bypass the restrictions exposed by sysfs and use the
> perf_events API directly. 

Define bad? The only case where perf cares (from a kernel pov) is when
it can make the hardware melt and similar things. Programming the PMU in
a non-nonsensical but non-destructive way is fine, you get what you ask
for.