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:	Tue, 6 Nov 2012 00:08:03 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Alexander Duyck <alexander.h.duyck@...el.com>
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 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>.

-- 
 Kirill A. Shutemov
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