[musl] Re: Linux powerpc new system call instruction and ABI
dalias at libc.org
Thu May 20 01:36:55 AEST 2021
On Wed, May 19, 2021 at 10:22:05AM -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 :-)
Even if it were easy to change, the old ppc ABI with a separate error
indicator is much worse to use. In musl we paper over archs that do
this silliness by converting to a normal negated errno code. There are
literally no syscalls that need the ability to return negative values
in addition to error codes; historically there were one or two (I only
recall one fcntl command) but there were ways to disambiguate and
they're only fallbacks for ancient kernels nowadays, if used at all.
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