linux-kernel - Re: [RFC PATCH 2/7] perf: add pipe-specific header read/write and event processing code

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Message-ID: <20100327230505.GK23594@ghostprotocols.net>
Date:	Sat, 27 Mar 2010 20:05:05 -0300
From:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Tom Zanussi <tzanussi@...il.com>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, rostedt@...dmis.org, k-keiichi@...jp.nec.com
Subject: Re: [RFC PATCH 2/7] perf: add pipe-specific header read/write and
 event processing code

Em Sat, Mar 27, 2010 at 07:57:47PM -0300, Arnaldo Carvalho de Melo escreveu:
> Em Sat, Mar 27, 2010 at 04:14:57AM +0100, Frederic Weisbecker escreveu:
> > On Wed, Mar 03, 2010 at 01:05:24AM -0600, Tom Zanussi wrote:
> > >  
> > > +int perf_header__write_pipe(struct perf_header *self, int fd)
> > > +{
> > > +	struct perf_file_header_pipe f_header;
> > > +	int err;
> > > +
> > > +	f_header = (struct perf_file_header_pipe){
> > > +		.magic	   = PERF_MAGIC,
> > > +		.size	   = sizeof(f_header),
> > > +	};
> 
> > I actually wonder why you bother with a header in the pipe mode,
> > since it only contains MAGIC and its size.
> 
> Because he wants to detect endianness  :-)

Elaborating: piping over netcat from a big endian to a little endian
machine works because he used the ->needs_swap logic I implemented to
allow cross platform analysis.

Its just a matter of having the DSOS with matching build-ids in the
cache of the machine running 'perf report'.

- Arnaldo
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