Re: [RFC PATCH v3 08/12] lib: vdso: allow arches to provide vdso data pointer

[Christophe Leroy]

Thomas, Andy,

Le 15/01/2020 Ã 07:15, Christophe Leroy a ÃcritÂ:
> Le 15/01/2020 Ã 00:06, Thomas Gleixner a ÃcritÂ:
> > Christophe Leroy <christophe.leroy@xxxxxx> writes:
> > > Â static __maybe_unused int
> > > +#ifdef VDSO_GETS_VD_PTR_FROM_ARCH
> > > +__cvdso_clock_gettime_common(const struct vdso_data *vd, clockid_t 
> > > clock,
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂ struct __kernel_timespec *ts)
> > > +{
> > > +#else
> > > Â __cvdso_clock_gettime_common(clockid_t clock, struct 
> > > __kernel_timespec *ts)
> > > Â {
> > > ÂÂÂÂÂ const struct vdso_data *vd = __arch_get_vdso_data();
> > > +#endif
> > > ÂÂÂÂÂ u32 msk;
> >
> > If we do that, then there is no point in propagating this to the inner
> > functions. It's perfectly fine to have this distinction at the outermost
> > level.
>
> In v2, I did it at the arch level (see 
> https://patchwork.ozlabs.org/patch/1214983/). Andy was concerned about 
> it being suboptimal for arches which (unlike powerpc) have PC related 
> data addressing mode.
>
> Wouldn't it be the same issue if doing it at the outermost level of 
> generic VDSO ?

Any opinion on this ?

From your point of view, what should I do:
A/ __arch_get_vdso_data() handled entirely at arch level and arches 
handing over the vdso data pointer to generic C VDSO functions all the 
time (as in my v2 series) ?
B/ Data pointer being handed over all the way up for arches wanting to 
do so, no changes at all for others (as in my v3 series) ?
C/ __arch_get_vdso_data() being called at the outermost generic level 
for arches not interested in handling data pointer from the caller (as 
suggested by Thomas) ?

Andy, with A/ you were concerned about arches being able to do PC 
related accesses. Would it be an issue for C/ as well ? If not, I guess 
C/ would be cleaner than B/ allthought not as clean as A which doesn't 
add any #ifdefery at all.

Thanks
Christophe