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

[Christophe Leroy]

Le 09/11/2023 à 11:18, Michael Ellerman a écrit :
> "Arnd Bergmann" <arnd@xxxxxxxx> writes:
>> 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.
> 
> AFAIK the ABI is just the symbols we export, as defined in the linker
> script:
> 
> /*
>   * This controls what symbols we export from the DSO.
>   */
> VERSION
> {
> 	VDSO_VERSION_STRING {
> 	global:
> 		__kernel_get_syscall_map;
> 		__kernel_gettimeofday;
> 		__kernel_clock_gettime;
> 		__kernel_clock_getres;
> 		__kernel_get_tbfreq;
> 		__kernel_sync_dicache;
> 		__kernel_sigtramp_rt64;
> 		__kernel_getcpu;
> 		__kernel_time;
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/powerpc/kernel/vdso/vdso64.lds.S?h=v6.6&#n117
> 
>> 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.
> 
> The __c symbols are not part of the ABI, so we could rename them.
> 
> At the moment though they don't have the same prototype as the generic
> versions, because we find the VDSO data in asm and pass it to the C
> functions, eg:
> 
> int __c_kernel_gettimeofday(struct __kernel_old_timeval *tv, struct timezone *tz,
> 			    const struct vdso_data *vd);
> 
> I think we can rework that though, by implementing
> __arch_get_vdso_data() and getting the vdso_data in C. Then we'd be able
> to share the prototypes.

I think it would not a been good idea, it would be less performant, for 
explanation see commit 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e876f0b69dc993e86ca7795e63e98385aa9a7ef3

Christophe