linux-kernel - Re: [PATCH v3 4/8] powerpc/vdso32: inline __get

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Date:   Wed, 27 Nov 2019 15:47:56 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v3 4/8] powerpc/vdso32: inline __get_datapage()

Hi Michael,

Le 22/11/2019 à 07:38, Michael Ellerman a écrit :
> Michael Ellerman <mpe@...erman.id.au> writes:
>> Christophe Leroy <christophe.leroy@....fr> writes:
>>> __get_datapage() is only a few instructions to retrieve the
>>> address of the page where the kernel stores data to the VDSO.
>>>
>>> By inlining this function into its users, a bl/blr pair and
>>> a mflr/mtlr pair is avoided, plus a few reg moves.
>>>
>>> The improvement is noticeable (about 55 nsec/call on an 8xx)
>>>
>>> vdsotest before the patch:
>>> gettimeofday:    vdso: 731 nsec/call
>>> clock-gettime-realtime-coarse:    vdso: 668 nsec/call
>>> clock-gettime-monotonic-coarse:    vdso: 745 nsec/call
>>>
>>> vdsotest after the patch:
>>> gettimeofday:    vdso: 677 nsec/call
>>> clock-gettime-realtime-coarse:    vdso: 613 nsec/call
>>> clock-gettime-monotonic-coarse:    vdso: 690 nsec/call
>>>
>>> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
>>
>> This doesn't build with gcc 4.6.3:
>>
>>    /linux/arch/powerpc/kernel/vdso32/gettimeofday.S: Assembler messages:
>>    /linux/arch/powerpc/kernel/vdso32/gettimeofday.S:41: Error: unsupported relocation against __kernel_datapage_offset
>>    /linux/arch/powerpc/kernel/vdso32/gettimeofday.S:86: Error: unsupported relocation against __kernel_datapage_offset
>>    /linux/arch/powerpc/kernel/vdso32/gettimeofday.S:213: Error: unsupported relocation against __kernel_datapage_offset
>>    /linux/arch/powerpc/kernel/vdso32/gettimeofday.S:247: Error: unsupported relocation against __kernel_datapage_offset
>>    make[4]: *** [arch/powerpc/kernel/vdso32/gettimeofday.o] Error 1
> 
> Actually I guess it's binutils, which is v2.22 in this case.
> 
> Needed this:
> 
> diff --git a/arch/powerpc/include/asm/vdso_datapage.h b/arch/powerpc/include/asm/vdso_datapage.h
> index 12785f72f17d..0048db347ddf 100644
> --- a/arch/powerpc/include/asm/vdso_datapage.h
> +++ b/arch/powerpc/include/asm/vdso_datapage.h
> @@ -117,7 +117,7 @@ extern struct vdso_data *vdso_data;
>   .macro get_datapage ptr, tmp
>   	bcl	20, 31, .+4
>   	mflr	\ptr
> -	addi	\ptr, \ptr, __kernel_datapage_offset - (.-4)
> +	addi	\ptr, \ptr, (__kernel_datapage_offset - (.-4))@l
>   	lwz	\tmp, 0(\ptr)
>   	add	\ptr, \tmp, \ptr
>   .endm
> 

Are you still planning to getting this series merged ? Do you need any 
help / rebase / re-spin ?

Christophe