linux-kernel - Re: [PATCH v4 6/8] x86/ftrace: Use __pa_symbol instead of _

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Date:	Fri, 16 Nov 2012 17:20:56 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Alexander Duyck <alexander.h.duyck@...el.com>
Cc:	tglx@...utronix.de, mingo@...hat.com, andi@...stfloor.org,
	hpa@...or.com, Frederic Weisbecker <fweisbec@...il.com>,
	x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 6/8] x86/ftrace: Use __pa_symbol instead of __pa on C
 visible symbols

On Fri, 2012-11-16 at 13:57 -0800, Alexander Duyck wrote:
> Instead of using __pa which is meant to be a general function for converting
> virtual addresses to physical addresses we can use __pa_symbol which is the
> preferred way of decoding kernel text virtual addresses to physical addresses.
> 
> In this case we are not directly converting C visible symbols however if we
> know that the instruction pointer is somewhere between _text and _etext we
> know that we are going to be translating an address form the kernel text
> space.

Can I ask what the purpose of this is? I'm a little skeptical of a
change that is just "the preferred way".

Was there some bug that this fixes?

-- Steve

> 
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Frederic Weisbecker <fweisbec@...il.com>
> Signed-off-by: Alexander Duyck <alexander.h.duyck@...el.com>
> ---
>  arch/x86/kernel/ftrace.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
> index 1d41402..42a392a 100644
> --- a/arch/x86/kernel/ftrace.c
> +++ b/arch/x86/kernel/ftrace.c
> @@ -89,7 +89,7 @@ do_ftrace_mod_code(unsigned long ip, const void *new_code)
>  	 * kernel identity mapping to modify code.
>  	 */
>  	if (within(ip, (unsigned long)_text, (unsigned long)_etext))
> -		ip = (unsigned long)__va(__pa(ip));
> +		ip = (unsigned long)__va(__pa_symbol(ip));
>  
>  	return probe_kernel_write((void *)ip, new_code, MCOUNT_INSN_SIZE);
>  }
> @@ -279,7 +279,7 @@ static int ftrace_write(unsigned long ip, const char *val, int size)
>  	 * kernel identity mapping to modify code.
>  	 */
>  	if (within(ip, (unsigned long)_text, (unsigned long)_etext))
> -		ip = (unsigned long)__va(__pa(ip));
> +		ip = (unsigned long)__va(__pa_symbol(ip));
>  
>  	return probe_kernel_write((void *)ip, val, size);
>  }


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