linux-kernel - Re: perf: record segfaults for cycles event when collecting data on a VM

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Message-ID: <20120209111451.GM22598@amd.com>
Date:	Thu, 9 Feb 2012 12:14:51 +0100
From:	Joerg Roedel <joerg.roedel@....com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	David Ahern <dsahern@...il.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	LKML <linux-kernel@...r.kernel.org>,
	Jason Wang <jasowang@...hat.com>
Subject: Re: perf: record segfaults for cycles event when collecting data on
 a VM

On Thu, Feb 09, 2012 at 08:30:24AM +0100, Ingo Molnar wrote:
> 
> * Joerg Roedel <joerg.roedel@....com> wrote:
> 
> > > which makes sense. It forces 
> > > perf_session__find_machine_for_cpumode() to return the host 
> > > machine always.
> > 
> > Great, thanks. I will send two patches tomorrow to fix Jason's 
> > problem and change the default for perf_guest.
> 
> Well, if the crash is fixed then the the default can stay, 
> right?

David's crash is fixed by changing the default back to its original
value :)

> Generally we should treat all input data in a perf.data or even 
> the bits we get in the ring-buffer as external data that has to 
> be checked carefully, with no assumptions made about data.

Well, there are two options:

	1) Make sure machine == NULL does not happen. Changing the
	   default of perf_guest back to false does exactly this for
	   David's problem.
	2) Make sure that a machine == NULL pointer is never
	   dereferenced

I was going to fix it with option 1. Do you suggest option 2 is better?


	Joerg

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