Re: Debian's 32-bit glibc vs AT_SYSINFO
On 2015-09-24 15:15, Andy Lutomirski wrote:
> On Thu, Sep 24, 2015 at 3:12 PM, Aurelien Jarno <aurelien@aurel32.net> wrote:
> > On 2015-09-24 14:19, Andy Lutomirski wrote:
> >> Hi-
> >>
> >> It looks like 32-bit x86 Debian glibc doesn't use AT_SYSINFO. This
> >> kills syscall performance. Upstream glibc as well as Fedora's glibc
> >> work fine.
> >>
> >> This test:
> >>
> >> https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=x86/entry_compat&id=9d742073d4c0449b262dbe0a3dab4a7c1d831500
> >>
> >> inadvertently found this issue.
> >>
> >> Any ideas? I'm only barely a Debian user -- I have a Debian-based test VM.
> >
> > By default, the 32-bit glibc on Debian is built for i586, which does not
> > have the sysenter/syscall instructions. Therefore it doesn't use
> > AT_SYSINFO.
>
> I find this a bit odd. glibc isn't supposed to have those
> instructions, and i586 kernels will still provide AT_SYSINFO (I
> think), and it'll still work. Is this a Debian-specific thing?
Yes, i586 kernels provide AT_SYSINFO, but it can't be used by the glibc,
as int 0x80 has to be used for syscall on i586 processors. It's nothing
specific to Debian, it's like that in upstream glibc. See:
https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/i386/i686/dl-sysdep.h
and
https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/i386/dl-sysdep.h
Note that the first file is not used when compiling for i586 or lower,
while the second is used in both case
Aurelien
--
Aurelien Jarno GPG: 4096R/1DDD8C9B
aurelien@aurel32.net http://www.aurel32.net
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