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

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Message-ID: <20140121112004.GH30706@mudshark.cambridge.arm.com>
Date:	Tue, 21 Jan 2014 11:20:04 +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>,
	Steve Capper <steve.capper@...aro.org>
Subject: Re: [PATCH] ARM64: perf: support dwarf unwinding in compat mode

On Mon, Jan 20, 2014 at 05:05:14PM +0000, Jean Pihet wrote:
> Hi Will,
> 
> Here is an updated version of the change, which uses compat_sp at only
> one place.
> The drawback is that compat_user_mode is checked when calling
> compat_user_stack_pointer, which seems unnecessary. Unfortunately the
> check is not optimized out by the complier as I could check with
> objdump -S.
> 
> What do you think?

I think that's cleaner and really wouldn't worry about the couple of extra
instructions.

Cheers,

Will

> diff --git a/arch/arm64/include/asm/compat.h b/arch/arm64/include/asm/compat.h
> index fda2704..e71f81f 100644
> --- a/arch/arm64/include/asm/compat.h
> +++ b/arch/arm64/include/asm/compat.h
> @@ -228,7 +228,7 @@ static inline compat_uptr_t ptr_to_compat(void __user *uptr)
>         return (u32)(unsigned long)uptr;
>  }
> 
> -#define compat_user_stack_pointer() (current_pt_regs()->compat_sp)
> +#define compat_user_stack_pointer() (user_stack_pointer(current_pt_regs()))
> 
>  static inline void __user *arch_compat_alloc_user_space(long len)
>  {
> diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h
> index fbb0020..86d5b54 100644
> --- a/arch/arm64/include/asm/ptrace.h
> +++ b/arch/arm64/include/asm/ptrace.h
> @@ -133,7 +133,7 @@ struct pt_regs {
>         (!((regs)->pstate & PSR_F_BIT))
> 
>  #define user_stack_pointer(regs) \
> -       ((regs)->sp)
> +       (!compat_user_mode(regs)) ? ((regs)->sp) : ((regs)->compat_sp)
> 
>  /*
>   * Are the current registers suitable for user mode? (used to maintain
--
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