Possible regression by ab037dd87a2f (powerpc/vdso: Switch VDSO to generic C implementation.)

[Benjamin Herrenschmidt]

On Mon, 2021-08-02 at 16:18 +1000, Michael Ellerman wrote:
> 
> But to be fair the ABI of the VDSO has always been a little fishy,
> 
> because the entry points pretend to be a transparent wrapper around
> 
> system calls, but then in a case like this are not.

This is somewhat debatable :-) If your perspective is from an
application, whether your wrapper is glibc, the vdso or *** knows what
else, you can't make assumptions about the state of the registers on a
signal hitting somewhere in your call chain other than what's
guanranteed by the ABI overall (ie, TLS etc...).

Nowhere was it written that a VDSO call behaved strictly like an sc
instruction :-)
> 
> > Go up to this point has only used the vdso function __kernel_clock_gettime; it
> > is the only entry point which would need to explicitly avoid R30 for
> > Go's sake.
> 
> 
> I thought about limiting the workaround to just that code, but it seemed
> silly and likely to come back to bite us. Once the compilers decides to
> spill a non-volatile there are plenty of other ones to choose from.

Yeah fine graining this is a waste of time, I agree. Just stick with
fixing r30 for the whole vdso, it won't actually hurt, just make sure
this is somewhat documented as to why we do it (I don't remember what
you patch does off hand, I assume at least your git commit has all the
data, a comment near the offending line in the Makefile might be good
too).

Cheers,
Ben.