linux-kernel - Re: [PATCH v2 4/8] powerpc/vdso32: inline __get

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Message-ID: <87blvonwzz.fsf@santosiv.in.ibm.com>
Date:   Fri, 13 Sep 2019 19:01:28 +0530
From:   Santosh Sivaraj <santosh@...six.org>
To:     Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/8] powerpc/vdso32: inline __get_datapage()

Christophe Leroy <christophe.leroy@....fr> writes:

> Hi Santosh,
>
> Le 26/08/2019 à 07:44, Santosh Sivaraj a écrit :
>> Hi Christophe,
>> 
>> 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>
>>> ---
>>>   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 vdso32.so and vdso64.so 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?

Thanks,
Santosh