linux-kernel - Re: [PATCH v3 1/8] x86: Improve __phys_addr performance by making use of carry flags and inlining

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Date:	Mon, 05 Nov 2012 13:56:28 -0800
From:	Alexander Duyck <alexander.h.duyck@...el.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
CC:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	andi@...stfloor.org, linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v3 1/8] x86: Improve __phys_addr performance by making
 use of carry flags and inlining

On 11/05/2012 12:24 PM, Kirill A. Shutemov wrote:
> On Mon, Nov 05, 2012 at 11:04:06AM -0800, Alexander Duyck wrote:
>> This patch is meant to improve overall system performance when making use of
>> the __phys_addr call.  To do this I have implemented several changes.
>>
>> First if CONFIG_DEBUG_VIRTUAL is not defined __phys_addr is made an inline,
>> similar to how this is currently handled in 32 bit.  However in order to do
>> this it is required to export phys_base so that it is available if __phys_addr
>> is used in kernel modules.
>>
>> The second change was to streamline the code by making use of the carry flag
>> on an add operation instead of performing a compare on a 64 bit value.  The
>> advantage to this is that it allows us to significantly reduce the overall
>> size of the call.  On my Xeon E5 system the entire __phys_addr inline call
>> consumes a little less than 32 bytes and 5 instructions.  I also applied
>> similar logic to the debug version of the function.  My testing shows that the
>> debug version of the function with this patch applied is slightly faster than
>> the non-debug version without the patch.
>>
>> When building the kernel with the first two changes applied I saw build
>> warnings about __START_KERNEL_map and PAGE_OFFSET constants not fitting in
>> their type.  In order to resolve the build warning I changed their type from
>> UL to ULL.
> What kind of warning messages did you see?
> It's strange: sizeof(unsinged long) == sizeof(unsinged long long) on
> x86_64

One of the warnings is included below:

In file included from /usr/src/kernels/linux-next/arch/x86/include/asm/page_types.h:37,
                 from /usr/src/kernels/linux-next/arch/x86/include/asm/pgtable_types.h:5,
                 from /usr/src/kernels/linux-next/arch/x86/include/asm/boot.h:11,
                 from arch/x86/realmode/rm/../../boot/boot.h:26,
                 from arch/x86/realmode/rm/../../boot/regs.c:19,
                 from arch/x86/realmode/rm/regs.c:1:
/usr/src/kernels/linux-next/arch/x86/include/asm/page_64_types.h: In function '__phys_addr_nodebug':
/usr/src/kernels/linux-next/arch/x86/include/asm/page_64_types.h:63: warning: integer constant is too large for 'unsigned long' type
/usr/src/kernels/linux-next/arch/x86/include/asm/page_64_types.h:66: warning: integer constant is too large for 'unsigned long' type
/usr/src/kernels/linux-next/arch/x86/include/asm/page_64_types.h:66: warning: integer constant is too large for 'unsigned long' type

The warnings all seemed to originate from several different spots
throughout the x86 tree.  All of the warning messages include
arch/x86/boot/boot.h:26 and then from there up the included from list is
always the same.

Thanks,

Alex

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