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Date: Fri, 11 Aug 2017 16:53:30 +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>,
        Paul Turner <pjt@google.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Subject: Re: perf: multiple mmap of fd behavior on x86/ARM
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On Fri, Aug 11, 2017 at 12:06:39PM +0100, Mark Rutland wrote:
> On Fri, Aug 11, 2017 at 12:52:52PM +0200, Peter Zijlstra wrote:
> > 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.
> 
> It's a problem because access to unimplemented counters trap. So if a
> task gets migrated from a CPU with N counters to one with N-1, accessing
> counter N would be problematic.
> 
> So we'd need to account for that somehow, in addition to the usual
> sequence counter fun to verify the index was valid when the access was
> performed.

Aah, you need restartable-sequences :-)