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

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

On 10/22/19 12:25 PM, Florian Weimer wrote:
> * Stefan Liebler:
> 
>> As an svc invocation does not clobber any user space registers
>> despite of the return value r2 and it does not need a special
>> stack frame. This patch gets rid of the extra frame.
>> We just have to save and restore r6 and r7 as those are
>> preserved across function calls.
> 
> Looks okay to me.  Would it be possible to save r6 and r7 in
> caller-saved registers not clobbered by the system call?  That might
> provide another small benefit.
> 
> Thanks,
> Florian
> 
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.

Thanks,
Stefan