linux-kernel - Re: [patch] perf_event_open.2: 3.19 PERF_SAMPLE_REGS

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Message-ID: <alpine.DEB.2.11.1503021617260.17687@vincent-weaver-1.umelst.maine.edu>
Date:	Mon, 2 Mar 2015 16:19:15 -0500 (EST)
From:	Vince Weaver <vincent.weaver@...ne.edu>
To:	Stephane Eranian <eranian@...gle.com>
cc:	Jiri Olsa <jolsa@...hat.com>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	linux-man@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Chuck Ebbert <cebbert.lkml@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [patch] perf_event_open.2: 3.19 PERF_SAMPLE_REGS_INTR support

On Mon, 2 Mar 2015, Stephane Eranian wrote:

> vince,

> PEBS machine state. Problem is that there is only one set of pt_regs passed to
> __intel_pmu_pebs_event(). And if REGS_INTR is set, then the pt_regs
> registers are
> indeed overwritten with PEBS captured state. To avoid the issue, we
> would have to
> carry around two sets of pt_regs.

I don't think we have to carry around both (would that ever be useful?)
Just that the behavior is a bit surprising and I should document it in the 
manpage.

One question I do have: if it never makes sense to measure REGS_USER and 
REGS_INTR at the same time, why was the latter added at all?  Why not just 
have the REGS_USER information automatically upgrade to REGS_INTR if 
precise_ip is high enough?

Vince

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