Christophe Leroy [off-list ref] writes:

Le 09/11/2023 à 11:18, Michael Ellerman a écrit :

quoted

"Arnd Bergmann" [off-list ref] writes:

quoted

On Wed, Nov 8, 2023, at 19:31, Christophe Leroy wrote:

quoted

Le 08/11/2023 à 13:58, Arnd Bergmann a écrit :

quoted

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

quoted

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

Ah thanks. I was wondering why you had done it in asm.

It's a pity but you're right that's probably a measurable performance
hit for some of those calls.

cheers