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

Alexander Duyck
2012-11-16T19:35:56Z

On 11/05/2012 02:08 PM, Kirill A. Shutemov wrote:
> On Mon, Nov 05, 2012 at 01:56:28PM -0800, Alexander Duyck wrote:
>> 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.
> Realmode code compiles with -m32. I guess it's just wrong that it tries to
> include <asm/page_64_types.h>.
>

I have been reviewing things and I think the problem is due to the fact
that we have content in page_64_types.h that doesn't really belong
there. I will add a patch that moves the virtual to physical address
translation header contents over to page_64.h. That will keep it
consistent with where it is in the 32 bit build (page_32.h) and avoids
the build conflicts.

Expect a v4 of the patches with this fixed in the next few hours.

Thanks,

Alex