Linux powerpc new system call instruction and ABI

[Nicholas Piggin]

Excerpts from Dmitry V. Levin's message of May 20, 2021 11:06 am:
> On Wed, May 19, 2021 at 07:48:47PM -0400, Rich Felker wrote:
>> On Wed, May 19, 2021 at 06:09:25PM +0000, Joakim Tjernlund wrote:
> [...]
>> > 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.
> 
> Unfortunately, being a new independent interface doesn't mean it isn't
> an ABI break.  In fact, it was a severe ABI break, and this thread is
> an attempt to find a hotfix.

It is an ABI break, that was known. The ptrace info stuff I fixed with 
the patch earlier was obviously a bug in my initial implementation and 
not intended (sorry my ptrace testing was not sufficient, and thanks for
reporting it, by the way).

But the register ABI was always a known break. The issue is that rfscv
clobbers LR, so it can not support the old ABI. If the old ABI did not 
preserve LR, then we may have chosen to not change register ABI at all.

Thanks,
Nick