Messages in this thread | | | From | "Metzger, Markus T" <> | Subject | RE: [PATCH v8 02/14] perf: Add AUX area to ring buffer for raw data streams | Date | Mon, 17 Nov 2014 09:33:00 +0000 |
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> -----Original Message----- > From: Alexander Shishkin [mailto:alexander.shishkin@linux.intel.com] > Sent: Friday, November 14, 2014 2:44 PM > To: Peter Zijlstra > Cc: Ingo Molnar; linux-kernel@vger.kernel.org; Robert Richter; Frederic > Weisbecker; Mike Galbraith; Paul Mackerras; Stephane Eranian; Andi Kleen; > Liang, Kan; Hunter, Adrian; Metzger, Markus T; mathieu.poirier@linaro.org; > acme@infradead.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@infradead.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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