linux-kernel - Re: [PATCH] ARM64: perf: support dwarf unwinding in compat mode

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Message-ID: <20140117100723.GB16003@mudshark.cambridge.arm.com>
Date:	Fri, 17 Jan 2014 10:07:23 +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>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Arnaldo <acme@...stprotocols.net>,
	"patches@...aro.org" <patches@...aro.org>,
	Jiri Olsa <jolsa@...hat.com>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] ARM64: perf: support dwarf unwinding in compat mode

On Fri, Jan 17, 2014 at 09:00:09AM +0000, Jean Pihet wrote:
> On 16 January 2014 14:47, Jean Pihet <jean.pihet@...aro.org> wrote:
> >> So the simplest thing would be to make compat_user_stack_pointer expand to
> >> user_stack_pointer(current_pt_regs()) on arm64 and merge that in with your
> >> original patch fixing user_stack_pointer.
> 
> I see 2 issues in your proposal:
> 
> 1) user_stack_pointer(regs) calls compat_user_stack_pointer if
> compat_user_mode(regs)) and compat_user_stack_pointer expands to
> user_stack_pointer. I see a circular dependency in the macros.

Not today it doesn't, so you just need to avoid writing the circular
dependency and instead make user_stack_pointer access (regs)->compat_sp
instead.

> 2) current_pt_regs() returns the current task regs although perf
> passes a regs struct that had been recorded previously.

Yes, but compat_user_stack_pointer doesn't take a regs paramater anyway, so
there's no change in behaviour here.

Will
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