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

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Date:	Fri, 16 Nov 2012 17:45:26 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Alexander Duyck <alexander.h.duyck@...el.com>, 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

On Fri, 2012-11-16 at 14:25 -0800, H. Peter Anvin wrote:
> It is a performance improvement.
> 
> >>  	 * 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);
> >>  }
> 

#define __pa(x)		__phys_addr((unsigned long)(x))
#define __pa_symbol(x)	__pa(__phys_reloc_hide((unsigned long)(x)))


I'm confused. __pa_symbol() just calls __pa() with some macro magic to
its parameter. How is this a performance improvement?

-- Steve


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