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Date: Thu, 19 Dec 2013 11:26:25 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
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        Paul Mackerras <paulus@samba.org>,
        Stephane Eranian <eranian@google.com>, Andi Kleen <ak@linux.intel.com>
Subject: Re: [PATCH v0 04/71] itrace: Infrastructure for instruction flow
 tracing units
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On Thu, Dec 19, 2013 at 09:53:44AM +0200, Alexander Shishkin wrote:
> Peter Zijlstra <peterz@infradead.org> writes:
> > The thing is; why can't you zero-copy whatever buffer the hardware
> > writes into, into the normal buffer?
> 
> I'm not sure I understand. You mean, have the buffer split between perf
> data and trace data?

Yep, I don't see any reason why this wouldn't work.

When the hardware thing sends an interrupt to notify us its buffer is
'full', stop the recorder, try to create a single record in the buffer
that's big enough + 1 page, then swizzle the hardware pages and the
buffer pages for that record, using the +1 page to page align the actual
data. Then (re)start the hardware on the 'new' pages.