linux-kernel - Re: [PATCH 1/3] ARM64: perf: add support for perf registers API

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Date:	Wed, 15 Jan 2014 11:07:24 +0000
From:	Will Deacon <will.deacon@....com>
To:	Jean Pihet <jean.pihet@...aro.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Jiri Olsa <jolsa@...hat.com>, Ingo Molnar <mingo@...nel.org>,
	Arnaldo <acme@...stprotocols.net>,
	"patches@...aro.org" <patches@...aro.org>,
	Jean Pihet <jean.pihet@...oldbits.com>
Subject: Re: [PATCH 1/3] ARM64: perf: add support for perf registers API

On Wed, Jan 15, 2014 at 10:30:48AM +0000, Jean Pihet wrote:
> Hi Will,

Hi Jean,

> On 6 January 2014 19:30, Will Deacon <will.deacon@....com> wrote:
> > On Mon, Dec 30, 2013 at 04:25:30PM +0000, Jean Pihet wrote:
> >> From: Jean Pihet <jean.pihet@...oldbits.com>
> >>
> >> This patch implements the functions required for the perf registers API,
> >> allowing the perf tool to interface kernel register dumps with libunwind
> >> in order to provide userspace backtracing.
> >> Compat mode is also supported.
> >
> > [...]
> >
> >> +u64 perf_reg_value(struct pt_regs *regs, int idx)
> >> +{
> >> +     if (WARN_ON_ONCE((u32)idx >= PERF_REG_ARM64_MAX))
> >> +             return 0;
> >
> > While this is probably fine, I'd feel more comfortable if you had separate
> > limit checks for native and compat...
> In fact in the native and compat modes the same set of registers are
> accessed, based on the native regs that are registered to the perf
> event core, cf. the definition of PERF_REGS_MASK in
> tools/perf/arch/arm64/include/perf_regs.h.
> 
> The regs set could be registered differently based on the binary to
> trace, but unfortunately the perf core code does not allow that.
> 
> I would leave the code as is, what do you think?

Well, what business would a compat task have accessing registers beyond the
compat subset? Since we don't expose the PC, we can simply lower the limit
as the compat registers form a prefix of the native registers, no?

Will
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