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

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Date:	Mon, 17 Nov 2014 09:33:00 +0000
From:	"Metzger, Markus T" <markus.t.metzger@...el.com>
To:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	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>,
	Peter Zijlstra <peterz@...radead.org>
Subject: RE: [PATCH v8 02/14] perf: Add AUX area to ring buffer for raw data
 streams

> -----Original Message-----
> From: Alexander Shishkin [mailto:alexander.shishkin@...ux.intel.com]
> Sent: Friday, November 14, 2014 2:44 PM
> To: Peter Zijlstra
> Cc: Ingo Molnar; linux-kernel@...r.kernel.org; Robert Richter; Frederic
> Weisbecker; Mike Galbraith; Paul Mackerras; Stephane Eranian; Andi Kleen;
> Liang, Kan; Hunter, Adrian; Metzger, Markus T; mathieu.poirier@...aro.org;
> acme@...radead.org; Peter Zijlstra; Alexander Shishkin
> Subject: [PATCH v8 02/14] perf: Add AUX area to ring buffer for raw data
> streams
> 
> 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?

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

Regards,
Markus.

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