linux-kernel - Re: [PATCH 1/1] MM: virtual address debug

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Date:	Wed, 18 Jun 2008 15:41:52 +0200
From:	Jiri Slaby <jirislaby@...il.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
CC:	Ingo Molnar <mingo@...hat.com>, tglx@...utronix.de, hpa@...or.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH 1/1] MM: virtual address debug

Nick Piggin napsal(a):
> On Thursday 12 June 2008 21:56, Jiri Slaby wrote:
>> Add some (configurable) expensive sanity checking to catch wrong address
>> translations on x86.
>>
>> - create linux/mmdebug.h file to be able include this file in
>>   asm headers to not get unsolvable loops in header files
>> - __phys_addr on x86_32 became a function in ioremap.c since
>>   PAGE_OFFSET, is_vmalloc_addr and VMALLOC_* non-constasts are undefined
>>   if declared in page_32.h
> 
> Uh, I have to disagree with this. __phys_addr is used in some really
> performance critical parts of the kernel, and the function calls are
> free mindset is just wrong. Even for modern x86 CPUs, the function
> call return might take 10 cycles or more when you include all costs.
> 
> And for something like this
> 
> #define __phys_addr(x)         ((x) - PAGE_OFFSET)
> 
> the code to call the function is probably bigger than inline generated
> code anyway.

Thanks for comments, well, are you OK with __phys_addr being a function only 
on CONFIG_VIRTUAL_DEBUG?
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