Re: [musl] Re: Linux powerpc new system call instruction and ABI

[Rich Felker <dalias@libc.org>]

On Wed, May 19, 2021 at 06:09:25PM +0000, Joakim Tjernlund wrote:
> On Wed, 2021-05-19 at 10:22 -0500, Segher Boessenkool wrote:
> > On Wed, May 19, 2021 at 03:06:49PM +0000, Joakim Tjernlund wrote:
> > > On Wed, 2021-05-19 at 09:38 -0500, Segher Boessenkool wrote:
> > > > On Wed, May 19, 2021 at 06:42:40PM +1000, Nicholas Piggin wrote:
> > > > > Excerpts from Joakim Tjernlund's message of May 19, 2021 6:08 pm:
> > > > > > I always figured the ppc way was superior. It begs the question if not the other archs should
> > > > > > change instead?
> > > > > 
> > > > > It is superior in some ways, not enough to be worth being different.
> > > > 
> > > > The PowerPC syscall ABI *requires* using cr0.3 for indicating errors,
> > > > you will have to do that whether you conflate the concepts of return
> > > > code and error indicator or not!
> > > > 
> > > > > Other archs are unlikely to change because it would be painful for
> > > > > not much benefit.
> > > > 
> > > > Other archs cannot easily change for much the same reason :-)
> > > 
> > > Really? I figured you could just add extra error indication in kernel syscall I/F.
> > > Eventually user space could migrate to the new indication.
> > 
> > You seem to assume all user space uses glibc, or *any* libc even?  This
> > is false.  Some programs do system calls directly.  Do not break the
> > kernel ABI :-)
> 
> Adding an extra indicator does not break ABI, does it ?

It does, because the old ABI on most archs has no clobbers except the
return value register. Some archs though have additional
syscall-clobbered regs that could be repurposed as extra return
values, but you could only rely on them being meaningful after probing
for a kernel that speaks the new variant. This just makes things more
complicated, not more useful.

> W.r.t breaking ABI, isn't that what PowerPC is trying to do with the new syscall I/F? 

No, it's a new independent interface.

Rich