linux-kernel - Re: [PATCH 3/3 v2] perf tool: give user better message if precise is not supported

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Date:	Fri, 14 Sep 2012 22:05:18 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	David Ahern <dsahern@...il.com>, linux-kernel@...r.kernel.org,
	Robert Richter <robert.richter@....com>
Subject: Re: [PATCH 3/3 v2] perf tool: give user better message if precise is
 not supported

* Peter Zijlstra <peterz@...radead.org> wrote:

> On Fri, 2012-09-14 at 11:00 -0700, Arnaldo Carvalho de Melo wrote:
> > > Understood and there have been suggestions on how to definitely state
> > > what the kernel side did not like. I like Peter's last suggestion --
> > > something along the lines of clearing attr on a failure except the
> > > offending setting.
> > 
> > I think ws need to use a new bit 
> 
> Quite so, for all the reasons you list. But you like the 
> general idea? I wasn't sure I did, but it was the only thing I 
> could come up with that would sort of do what we need it to.
> 
> The fact that you destroy the user input is awkward, I don't 
> think there's another syscall that behaves in this fashion.

Destroying/clearing stuff looks really hacky.

Why not use a single error status field, set via a long list of 
enum error constants, a 'perf errnos'?

The only real problem with the kernel's syscall error code is 
that it's not wide enough for historic reasons, so we cannot 
just create our own errnos. But we can create our errors in the 
attr just fine and make them finegrained enough so that tooling 
can figure out what happened exactly when it gets a syscall 
error.

Yes, that's old-fashioned technology, but it works. With time we 
could put some structure into the list of error IDs, to make it 
easily extensible yet grouped in some fashion, etc.

Thanks,

	Ingo
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