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Re: [PATCH] s390: optimize syscall function
- From: Martin Schwidefsky <schwidefsky at de dot ibm dot com>
- To: Adhemerval Zanella <adhemerval dot zanella at linaro dot org>
- Cc: Christian Borntraeger <borntraeger at de dot ibm dot com>, Stefan Liebler <stli at linux dot vnet dot ibm dot com>, Andreas Krebbel <krebbel at linux dot vnet dot ibm dot com>, libc-alpha at sourceware dot org
- Date: Mon, 12 Jun 2017 15:55:05 +0200
- Subject: Re: [PATCH] s390: optimize syscall function
- Authentication-results: sourceware.org; auth=none
- References: <1497007747-191015-1-git-send-email-borntraeger@de.ibm.com> <20170612074958.71090d0f@mschwideX1> <fe60b8c0-7105-0c9a-f62e-0e5540308d31@linaro.org>
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.
--
blue skies,
Martin.
"Reality continues to ruin my life." - Calvin.