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

[Stefan Liebler <stli@linux.ibm.com>]

On 10/22/19 1:44 PM, Florian Weimer wrote:
> * 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
> 

Committed.
Thanks