This is the mail archive of the
libc-alpha@sourceware.org
mailing list for the glibc project.
Re: [PATCH] s390: optimize syscall function
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?
--
blue skies,
Martin.
"Reality continues to ruin my life." - Calvin.