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

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Message-ID: <875zlwnvio.fsf@santosiv.in.ibm.com>
Date:   Fri, 13 Sep 2019 19:33:27 +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:

> Le 13/09/2019 à 15:31, Santosh Sivaraj a écrit :
>> 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?
>> 
>
> 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
> 	help
> 	  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.
>

I didn't know we build 32 bit vdso for 64 bit big endians. But I don't think
its difficult to do it, might be a bit tricky. We can have two targets from
the same source.

SRC = vdso/*.c
OBJS_32 = $(SRC:.c=vdso32/.o)
OBJS_64 = $(SRC:.c=vdso64/.o)

Something like this would work. Of course, this is out of memory, might have to
do something slightly different for the Makefiles in kernel.

Thanks,
Santosh

>
>
>
> Christophe