Re: [PATCH v2 4/8] powerpc/vdso32: inline __get_datapage()

From: Christophe Leroy
Date: Fri Sep 13 2019 - 09:50:11 EST

Le 13/09/2019 Ã 15:31, Santosh Sivaraj a ÃcritÂ:
Christophe Leroy <christophe.leroy@xxxxxx> writes:

Hi Santosh,

Le 26/08/2019 Ã 07:44, Santosh Sivaraj a ÃcritÂ:
Hi Christophe,

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>
arch/powerpc/kernel/vdso32/cacheflush.S | 10 +++++-----
arch/powerpc/kernel/vdso32/datapage.S | 29 ++++-------------------------
arch/powerpc/kernel/vdso32/datapage.h | 11 +++++++++++
arch/powerpc/kernel/vdso32/gettimeofday.S | 13 ++++++-------
4 files changed, 26 insertions(+), 37 deletions(-)
create mode 100644 arch/powerpc/kernel/vdso32/datapage.h

The datapage.h file should ideally be moved under include/asm, then we can use
the same for powerpc64 too.

I have a more ambitious project indeed.

Most of the VDSO code is duplicated between vdso32 and vdso64. I'm
aiming at merging everything into a single source code.

This means we would have to generate and out of the
same source files. Any idea on how to do that ? I'm not too good at
creating Makefiles. I guess we would have everything in
arch/powerpc/kernel/vdso/ and would have to build the objects twice,
once in arch/powerpc/kernel/vdso32/ and once in arch/powerpc/kernel/vdso64/

Should we need to build the objects twice? For 64 bit config it is going to be
a 64 bit build else a 32 bit build. It should suffice to get the single source
code compile for both, maybe with macros or (!)CONFIG_PPC64 conditional
compilation. Am I missing something when you say build twice?

IIUC, on PPC64 we build vdso64 for 64bits user apps and vdso32 for 32bits user apps.

In arch/powerpc/kernel/Makefile, you have:

obj-$(CONFIG_VDSO32) += vdso32/
obj-$(CONFIG_PPC64) += vdso64/

And in arch/powerpc/platforms/Kconfig.cputype, you have:

config VDSO32
def_bool y
depends on PPC32 || CPU_BIG_ENDIAN
This symbol controls whether we build the 32-bit VDSO. We obviously
want to do that if we're building a 32-bit kernel. If we're building
a 64-bit kernel then we only want a 32-bit VDSO if we're building for
big endian. That is because the only little endian configuration we
support is ppc64le which is 64-bit only.