Re: [PATCH] s390: optimize syscall function

[Adhemerval Zanella <adhemerval dot zanella at linaro dot org>]

> On 13 Jun 2017, at 13:01, Martin Schwidefsky <schwidefsky@de.ibm.com> wrote:
> 
> On Tue, 13 Jun 2017 11:31:09 -0300
> Adhemerval Zanella <adhemerval.zanella@linaro.org> wrote:
> 
>>> On 12/06/2017 10:55, Martin Schwidefsky wrote:
>>> On Mon, 12 Jun 2017 09:04:36 -0300
>>> Adhemerval Zanella <adhemerval.zanella@linaro.org> wrote:
>>> 
>>>>> On 12/06/2017 02:49, Martin Schwidefsky wrote:  
>>>>> On Fri,  9 Jun 2017 13:29:07 +0200
>>>>> Christian Borntraeger <borntraeger@de.ibm.com> wrote:
>>>>> 
>>>>>> Since kernel 2.6.0 all Linux version accept the system call number
>>>>>> in register 1 for svc 0. There is no need to have special handling
>>>>>> that uses EX for system calls < 256. This will simplify and speed
>>>>>> up that code.
>>>>>> 
>>>>>> A microbenchmark doing "syscall(__NR_getpid);" in a loops gets faster
>>>>>> by ~12%.
>>>>>> 
>>>>>>        * sysdeps/unix/sysv/linux/s390/s390-64/syscall.S: Simplify
>>>>>>        code by always using SVC 0 instead of EX.
>>>>>>        * sysdeps/unix/sysv/linux/s390/s390-64/syscall.S: Likewise.
>>>>>> 
>>>>>> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
>>>>>> ---
>>>>>> sysdeps/unix/sysv/linux/s390/s390-32/syscall.S |   12 +++---------
>>>>>> sysdeps/unix/sysv/linux/s390/s390-64/syscall.S |   12 +++---------
>>>>>> 2 files changed, 6 insertions(+), 18 deletions(-)    
>>>>> 
>>>>> NAK. E.g. this from glibc:
>>>>> 
>>>>> sysdeps/unix/sysv/linux/s390/s390-64:
>>>>> 
>>>>> ENTRY (syscall)
>>>>>    ...
>>>>>        basr   %r8,0
>>>>> 0:      clg    %r1,4f-0b(%r8)      /* svc number < 256? */
>>>>>        jl     2f
>>>>> 1:      svc    0
>>>>>        j      3f
>>>>> 2:      ex     %r1,1b-0b(%r8)      /* lsb of R1 is subsituted as SVC number */
>>>>> 3:      lg     %r15,0(%r15)        /* load back chain */
>>>>>        cfi_adjust_cfa_offset (-160)
>>>>>        lmg     %r6,15,48(%r15)    /* Load registers.  */
>>>>>    ...
>>>>> 
>>>>> And there are old version of glibc where NR_syscalls was < 256 that
>>>>> exclusively used the SVC instruction with the system call number
>>>>> encoded in the instruction itself.
>>>>> 
>>>> 
>>>> I am not following the NAK, is the 'svc' way not really allowed for current
>>>> minimum kernel supported for s390 (3.2) or is the 'svc' method not really the 
>>>> fastest one for such syscalls?  
>>> 
>>> I claim lack of coffee after the long weekend ..
>>> 
>>> Yes, with version 3.2 as the minimum kernel level for using a glibc with
>>> the prosed patch everything should be fine. The support for system calls
>>> larger than 256 has been added with 2.5.68.  
>> 
>> Thanks, since it should be ok we can also cleanup the {INLINE,INTERNAL}_SYSCALL
>> macros in s390x sysdep.h as well.
> 
> How so? If you have a fixed system call number smaller than 256 the best option
> is to encode the system call number in the instruction. Loading the number to
> %r1 and then doing SVC-0 requires an additional instruction, no?

Indeed, I had the wrong assumption the syscall.S method was used in sysdep.h.