linux-kernel - Re: [PATCH v8 02/14] perf: Add AUX area to ring buffer for raw data streams

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Date:	Fri, 9 Jan 2015 16:18:14 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Metzger, Markus T" <markus.t.metzger@...el.com>
Cc:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Robert Richter <rric@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>,
	Andi Kleen <ak@...ux.intel.com>,
	"Liang, Kan" <kan.liang@...el.com>,
	"Hunter, Adrian" <adrian.hunter@...el.com>,
	"mathieu.poirier@...aro.org" <mathieu.poirier@...aro.org>,
	"acme@...radead.org" <acme@...radead.org>
Subject: Re: [PATCH v8 02/14] perf: Add AUX area to ring buffer for raw data
 streams

On Mon, Nov 17, 2014 at 09:33:00AM +0000, Metzger, Markus T wrote:
> > From: Peter Zijlstra <peterz@...radead.org>
> > 
> > This patch introduces "AUX space" in the perf mmap buffer, intended for
> > exporting high bandwidth data streams to userspace, such as instruction
> > flow traces.
> > 
> > AUX space is a ring buffer, defined by aux_{offset,size} fields in the
> > user_page structure, and read/write pointers aux_{head,tail}, which abide
> > by the same rules as data_* counterparts of the main perf buffer.
> > 
> > In order to allocate/mmap AUX, userspace needs to set up aux_offset to
> > such an offset that will be greater than data_offset+data_size and
> > aux_size to be the desired buffer size. Both need to be page aligned.
> > Then, same aux_offset and aux_size should be passed to mmap() call and
> > if everything adds up, you should have an AUX buffer as a result.
> 
> Why would userspace need to fill in aux_offset and aux_size?  Can't the kernel
> do this when userspace mmaps the aux buffer like it does for the respective
> data_* fields?

I suppose we could; I'm trying to remember why I did it like this, I'm
failing to remember much past yesterday atm :/ 

> How would userspace fill in those fields if it mapped the data buffer read-only?

Good point, its not used much atm. But yes. That also reminds me; we
should probably finish this:

  https://lkml.org/lkml/2013/7/8/154
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