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

[Metzger, Markus T]

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