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Date:	Mon, 10 Nov 2014 11:27:25 +0100
From:	Ingo Molnar <>
To:	Arnaldo Carvalho de Melo <>
Cc:	Peter Zijlstra <>,
	Vince Weaver <>,
	Stephane Eranian <>,
	Jiri Olsa <>,
	Andy Lutomirski <>,
	Thomas Gleixner <>,
	LKML <>
Subject: Re: [RFD] perf syscall error handling

* Arnaldo Carvalho de Melo <> wrote:

> Em Mon, Nov 03, 2014 at 05:50:19PM +0100, Peter Zijlstra escreveu:
> > On Mon, Nov 03, 2014 at 02:25:48PM -0200, Arnaldo Carvalho de Melo wrote:
> > > The way that peterz suggested, i.e. returning information about which
> > > perf_event_attr and which of the parameters was invalid/had issues could
> > > help with fallbacking/capability querying, i.e. tooling may want to use
> > > some features if available automagically, fallbacking to something else
> > > when that fails.
> > > We already do that to some degree in various cases, but for some if the
> > > only way that becomes available to disambiguate some EINVAL return is a
> > > string, code will start having strcmps :-\
> > OK, so how about we do both, the offset+mask for the tools 
> > and the string for the humans?
> Yeah, tooling tries to provide the best it can with the 
> offset+mask, and if doesn't manage to do anything smart with 
> it, just show the string and hope that helps the user to figure 
> out what is happening.

Almost: tooling should generally always consider the string as 
well, for the (not so uncommon) case where there can be multiple 
problems with the same field.

Really, I think the string will give the most bang for the buck, 
because it's really simple and straightforward on the kernel side 
(so that we have a good chance of achieving full coverage 
relatively quickly), and later on we could still complicate it 
all with offset+mask if there's really a need.

So lets start with an error string...


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