linux-kernel - Re: [RFC PATCH 0/3]perf/core: extend perf_reg and perf_sample_regs

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Date:	Fri, 06 Nov 2015 20:39:59 +1100
From:	Michael Ellerman <mpe@...erman.id.au>
To:	Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>,
	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, x86@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Stephane Eranian <eranian@...il.com>,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [RFC PATCH 0/3]perf/core: extend perf_reg and
 perf_sample_regs_intr

On Fri, 2015-11-06 at 13:17 +0530, Madhavan Srinivasan wrote:
> On Friday 06 November 2015 08:28 AM, Sukadev Bhattiprolu wrote:
> > Peter Zijlstra [peterz@...radead.org] wrote:
> > > On Thu, Nov 05, 2015 at 02:16:15AM +0530, Madhavan Srinivasan wrote:
> > > > Second patch updates struct arch_misc_reg for arch/powerpc with pmu registers
> > > > and adds offsetof macro for the same. It extends perf_reg_value()
> > > > to use reg idx to decide on struct to return value from.
> > > 
> > > Why; what's in those regs?
> > 
> > Those are PMU control registers/counters (in Patch 2) that are of
> > interest only in the context of a PMU interrupt and not relevant
> > to ptrace itself.
> 
> Yes. Thats right.

> > Could we add those registers to 'struct pt_regs' anyway?
> 
> I would prefer not to. Since as you mentioned, these are
> not relevant to ptrace. Currently patch 2, adds only few
> pmu registers, but would like to include more.

You can't just add them to pt_regs, it's part of the userspace ABI.

We could define a kernel internal version of pt_regs, but I don't think we want
to for this.

If we did that would bloat pt_regs for all users in the kernel, when we really
only want these regs for perf.

cheers

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