Re: [PATCH] S390: Remove not needed stack frame in syscall function.

[Florian Weimer <fweimer@redhat.com>]

* Stefan Liebler:

> The syscall itself just clobbers the return value in r2. But for its
> parameters we have to clobber r6 and r7.
>
> According to the ABI, r0-r5 and r14 are volatile.
> We need r1 for the syscall number for "svc 0", r2-r7 as parameters for
> svc and r14 is the return-address.
>
> Thus we could use r0 for saving/restoring r6.
> For r7 we have the option to either use the register save area on the
> stack-frame prepared by the caller or one of the volatile fprs. But
> the instructions for transferring gpr <-> fpr are not available with
> all architecture level sets. Thus we would need something like #ifdef
> / #else to provide alternative implementations.
>
> Therefore I think just storing/restoring both registers at once with
> one stmg/lmg instruction is okay.

I see.  I didn't know the ABI details.

Thanks,
Florian