linux-kernel - Re: [PATCH 3/7] Revert "perf: Fix warning while reading ring buffer headers"

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Date:	Sat, 15 May 2010 05:39:35 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Ian Munsie <imunsie@....ibm.com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>, Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Tom Zanussi <tzanussi@...il.com>,
	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
Subject: Re: [PATCH 3/7] Revert "perf: Fix warning while reading ring
	buffer headers"

On Thu, May 13, 2010 at 04:03:48PM +1000, Ian Munsie wrote:
> From: Ian Munsie <imunsie@....ibm.com>
> 
> This reverts commit d00a47cce569a3e660a8c9de5d57af28d6a9f0f7.
>     "perf: Fix warning while reading ring buffer headers"
> 
> The reverted patch removed the processing of the header_page, skipping
> over it instead on the assumption that perf was not using any of the
> data from that header. The patch neglected to remove the header_page_
> variables which were initialised in the removed code, nor did it fix any
> code that was using those variables.
> 
> In particular, long_size was set based on one of those variables
> (header_page_size_size) to learn the size of a long from the kernel,
> which is necessary to correctly print out some of the trace information
> in some circumstances. For instance, the size of a long in a 64 bit
> kernel would differ from the size of a long in perf if it was compiled
> for a 32 bit userspace. Perf trace needs to know the size of a long in
> the kernel so that it can print out the correct value without
> truncation.



I don't understand.
In the format file we have the size of the fields beside their type name,
so why do we need this?

Thanks.

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