Re: [PATCH v3 4/8] powerpc/vdso32: inline __get_datapage()

From: Christophe Leroy
Date: Mon Dec 02 2019 - 03:02:17 EST




On 11/28/2019 05:31 AM, Michael Ellerman wrote:
Christophe Leroy <christophe.leroy@xxxxxx> writes:
Le 22/11/2019 Ã 07:38, Michael Ellerman a ÃcritÂ:
Michael Ellerman <mpe@xxxxxxxxxxxxxx> writes:
Christophe Leroy <christophe.leroy@xxxxxx> 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@xxxxxx>

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 ?

Not sure. I'll possibly send a 2nd pull request next week with it
included.


v3 conflicts with 2038 cleanup series from Arnd merged yesterday by Linus (ceb307474506 ("Merge tag 'y2038-cleanups-5.5' of git://git.kernel.org:/pub/scm/linux/kernel/git/arnd/playground"))

Not seen your Ack in that series.

I sent out v4 of my series, rebased on top of 2038 cleanup series.

Christophe