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

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Message-ID: <a162a888-9892-43ca-ad17-61c19936c6f4@email.android.com>
Date:	Fri, 16 Nov 2012 14:25:28 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Steven Rostedt <rostedt@...dmis.org>,
	Alexander Duyck <alexander.h.duyck@...el.com>
CC:	tglx@...utronix.de, mingo@...hat.com, andi@...stfloor.org,
	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

It is a performance improvement.

Steven Rostedt <rostedt@...dmis.org> wrote:

>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);
>>  }

-- 
Sent from my mobile phone. Please excuse brevity and lack of formatting.
--
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