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

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Date:	Fri, 6 Nov 2015 13:17:09 +0530
From:	Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>
To:	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>,
	Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [RFC PATCH 0/3]perf/core: extend perf_reg and
 perf_sample_regs_intr



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.
 
> We do have 'struct perf_regs' but that seems to be arch nuetral.
> If architectures could override that, maybe we could add these
> new registers there without touching 'struct pt_regs'.

Exactly, idea here is to capture more data using perf_sample_reg_intr
without extending pt_regs structure.

Maddy

> Even so, lot of perf code depends on 'struct pt_regs'.
>
> Sukadev

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