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

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Date:   Fri, 22 Nov 2019 17:38:57 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Christophe Leroy <christophe.leroy@....fr>,
        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()

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


cheers