linux-kernel - Re: [PATCH] x86/boot: drop memset from copy.S

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Date:   Tue, 08 Jan 2019 00:38:27 -0800
From:   hpa@...or.com
To:     Cao jin <caoj.fnst@...fujitsu.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
CC:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de
Subject: Re: [PATCH] x86/boot: drop memset from copy.S

On January 7, 2019 12:52:57 AM PST, Cao jin <caoj.fnst@...fujitsu.com> wrote:
>Hi,
>
>On 1/7/19 3:59 PM, hpa@...or.com wrote:
>> On January 6, 2019 11:40:56 PM PST, Cao jin
><caoj.fnst@...fujitsu.com> wrote:
>>> According to objdump output of setup, function memset is not used in
>>> setup code. Currently, all usage of memset in setup come from macro
>>> definition of string.h.
>>>
>>> Signed-off-by: Cao jin <caoj.fnst@...fujitsu.com>
>>> ---
>>> Compiled and booted under x86_64; compiled under i386.
>>>
>>> Questions: now there is 2 definition of memcpy, one lies in copy.S,
>>> another lies in string.h which is mapped to gcc builtin function. Do
>we
>>> still need 2 definition? Could we move the content of copy.S to
>>> boot/string.c?
>>>
>>> At first glance, the usage of string.{c.h} of setup is kind of
>>> confusing,
>>> they are also used in compressed/ and purgatory/
>>>
>>> arch/x86/boot/copy.S | 15 ---------------
>>> 1 file changed, 15 deletions(-)
>>>
>>> diff --git a/arch/x86/boot/copy.S b/arch/x86/boot/copy.S
>>> index 15d9f74b0008..5157d08b0ff2 100644
>>> --- a/arch/x86/boot/copy.S
>>> +++ b/arch/x86/boot/copy.S
>>> @@ -33,21 +33,6 @@ GLOBAL(memcpy)
>>> 	retl
>>> ENDPROC(memcpy)
>>>
>>> -GLOBAL(memset)
>>> -	pushw	%di
>>> -	movw	%ax, %di
>>> -	movzbl	%dl, %eax
>>> -	imull	$0x01010101,%eax
>>> -	pushw	%cx
>>> -	shrw	$2, %cx
>>> -	rep; stosl
>>> -	popw	%cx
>>> -	andw	$3, %cx
>>> -	rep; stosb
>>> -	popw	%di
>>> -	retl
>>> -ENDPROC(memset)
>>> -
>>> GLOBAL(copy_from_fs)
>>> 	pushw	%ds
>>> 	pushw	%fs
>> 
>> This is dependent on both gcc version and flags.
>> 
>
>Thanks for your info, but I still don't quite get your point. All files
>who has memset reference in setup will also #include "string.h", so how
>gcc version and flags will associate memset reference to the assembly
>function by force?  Hope to get a little more details when you are
>convenient:)

GCC will sometimes emit calls to certain functions including memcpy().
-- 
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