Date: Fri, 11 Aug 2017 12:52:52 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Vince Weaver <vincent.weaver@maine.edu>, linux-kernel@vger.kernel.org,
        Will Deacon <will.deacon@arm.com>, Ingo Molnar <mingo@redhat.com>
Subject: Re: perf: multiple mmap of fd behavior on x86/ARM
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On Fri, Aug 11, 2017 at 11:01:27AM +0100, Mark Rutland wrote:
> On Thu, Aug 10, 2017 at 02:48:52PM -0400, Vince Weaver wrote:
> > 
> > So I was working on my perf_event_tests on ARM/ARM64 (the end goal was to 
> > get ARM64 rdpmc support working, but apparently those patches never made 
> > it upstream?)
> 
> IIUC by 'rdpmc' you mean direct userspace counter access?
> 
> Patches for that never made it upstream. Last I saw, there were no
> patches in a suitable state for review.
> 
> There are also difficulties (e.g. big.LITTLE systems where the number of
> counters can differ across CPUs) which have yet to be solved.

How would that be a problem? The API gives an explicit index to use with
the 'rdpmc' instruction.