linux-kernel - Re: [PATCH 15/22] arch: vdso: consolidate gettime prototypes

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Message-ID: <9cbfbd59-0761-406d-e97e-75e83bc4e6d0@csgroup.eu>
Date:   Thu, 9 Nov 2023 07:50:58 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Arnd Bergmann <arnd@...db.de>, Arnd Bergmann <arnd@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
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Subject: Re: [PATCH 15/22] arch: vdso: consolidate gettime prototypes



Le 08/11/2023 à 20:37, Arnd Bergmann a écrit :
> On Wed, Nov 8, 2023, at 19:31, Christophe Leroy wrote:
>> Le 08/11/2023 à 13:58, Arnd Bergmann a écrit :
> 
>> powerpc has functions doing more or less the same, they are called
>> __c_kernel_clock_gettime() and alike with their prototypes siting in
>> arch/powerpc/include/asm/vdso/gettimeofday.h
>>
>> Should those prototypes be moved to include/vdso/gettime.h too and
>> eventually renamed, or are they considered too powerpc specific ?
> 
> I don't actually know, my initial interpretation was that
> these function names are part of the user ABI for the vdso,
> but I never looked closely enough at how vdso works to
> be sure what the actual ABI is.
> 
> If __c_kernel_clock_gettime() etc are not part of the user-facing
> ABI, I think renaming them for consistency with the other
> architectures would be best.
> 

User-facing ABI function is __kernel_clock_gettime(), defined in 
arch/powerpc/kernel/vdso/gettimeofday.S , see man vdso.
There is no prototype defined for it anywhere, obviously that's 
undetected because it is assembly. Should a prototype be added somewhere 
anyway ?

__kernel_clock_gettime() sets up a stack frame, retrieves the address of 
the datapage then calls __c_kernel_clock_gettime() which then calls 
__cvdso_clock_gettime_data() which is part of the generic CVDSO.

Maybe that's too different from what other architectures do ?

Christophe