Le 15/01/2020 à 00:06, Thomas Gleixner a écrit :
Christophe Leroy <christophe.le...@c-s.fr> writes:
static __maybe_unused int
+#ifdef VDSO_GETS_VD_PTR_FROM_ARCH
+__cvdso_clock_gettime_common(const struct vdso_data *vd, clockid_t clock,
+                     struct __kernel_timespec *ts)
+{
+#else
  __cvdso_clock_gettime_common(clockid_t clock, struct __kernel_timespec *ts)
  {
        const struct vdso_data *vd = __arch_get_vdso_data();
+#endif
        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 ?


As a related question, I noticed that you keep all that ASM voodoo in
the PPC specific code which provides the actual entry points. Is that
ASM code really still necessary? All current users of the generic VDSO
just do something like:

int __vdso_clock_gettime(clockid_t clock, struct __kernel_timespec *ts)
{
         return __cvdso_clock_gettime(clock, ts);
}

in the architecture code. Is there a reason why this can't work on PPC?

The problem with powerpc is that VDSO functions have to (just like system calls) set the SO bit in CR register in case of error, or clear it if no error. There is no way to do that from the C function, because there is no way to tell GCC to not play up with CR register on function return.

Refer discussion at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92769

Christophe

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