linux-kernel - Re: perf: rdpmc and PERF_EVENT_IOC

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Date:	Thu, 18 Aug 2016 00:32:25 -0400 (EDT)
From:	Vince Weaver <vincent.weaver@...ne.edu>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: perf: rdpmc and PERF_EVENT_IOC_RESET

On Wed, 17 Aug 2016, Peter Zijlstra wrote:

> Hurm.. I never considered using RESET with rdpmc(). The typical usage of
> rdpmc() I considered is something like:

I'm finally trying to hook up PAPI to properly use rdpmc and PAPI likes to 
use IOC_RESET in various places.  I can just special case to not do so 
when in rdpmc mode, but I got curious if RESET was expected to work.

> But the rdpmc user function should:
> ...
> 
> Which sign-extends the RDPMC result and adds it to userpg->offset,
> resulting in a positive number again.

I will have to check to see if PAPI is using the proper code.
What happens is the procesed result has bit 49 set for some reason, so the 
results are 2^49 too big.  I've been trying to track it down, but the PAPI 
code is a bit of a mess (to put it mildly).

Vince