[musl] Powerpc Linux 'scv' system call ABI proposal take 2

[Rich Felker]

On Thu, Apr 16, 2020 at 11:21:56AM -0400, Jeffrey Walton wrote:
> On Wed, Apr 15, 2020 at 8:17 PM Nicholas Piggin <npiggin at gmail.com> wrote:
> >
> > Excerpts from Rich Felker's message of April 16, 2020 8:55 am:
> > > On Thu, Apr 16, 2020 at 07:45:09AM +1000, Nicholas Piggin wrote:
> > >> I would like to enable Linux support for the powerpc 'scv' instruction,
> > >> as a faster system call instruction.
> > >>
> > >> This requires two things to be defined: Firstly a way to advertise to
> > >> userspace that kernel supports scv, and a way to allocate and advertise
> > >> support for individual scv vectors. Secondly, a calling convention ABI
> > >> for this new instruction.
> > >> ...
> > > Note that any libc that actually makes use of the new functionality is
> > > not going to be able to make clobbers conditional on support for it;
> > > branching around different clobbers is going to defeat any gains vs
> > > always just treating anything clobbered by either method as clobbered.
> >
> > Well it would have to test HWCAP and patch in or branch to two
> > completely different sequences including register save/restores yes.
> > You could have the same asm and matching clobbers to put the sequence
> > inline and then you could patch the one sc/scv instruction I suppose.
> 
> Could GCC function multiversioning work here?
> https://gcc.gnu.org/wiki/FunctionMultiVersioning
> 
> It seems like selecting a runtime version of a function is the sort of
> thing you are trying to do.

On glibc it potentially could. This is ifunc-based functionality
though and musl explicitly does not (and will not) support ifunc
because of lots of fundamental problems it entails. But even on glibc
the underlying mechanisms for ifunc are just the same as a normal
indirect call and there's no real reason to prefer implementing it
with ifunc/multiversioning vs directly.

Rich