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

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Date:	Fri, 16 Nov 2012 15:20:04 -0800
From:	Alexander Duyck <alexander.h.duyck@...el.com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Steven Rostedt <rostedt@...dmis.org>, 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 11/16/2012 03:06 PM, H. Peter Anvin wrote:
> On 11/16/2012 02:45 PM, Steven Rostedt wrote:
>>
>> #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?
>>
>
> One of the earlier patches in this series changes __pa_symbol() to
> avoid the conditional hidden inside __phys_addr(), since by definition
> a symbol can only be on one side of that branch.
>
>     -hpa
>

In addition to being a bit faster the code is also a bit smaller since
it can combine the the constants from __va() and __pa_symbol() as the
new __pa_symbol is an inline in the non-debug case.

Thanks,

Alex
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