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Re: FOR REVIEW: New x86-64 vsyscall vgetcpu()
- From: Andi Kleen <ak at suse dot de>
- To: Jakub Jelinek <jakub at redhat dot com>
- Cc: Zoltan Menyhart <Zoltan dot Menyhart at bull dot net>, Jes Sorensen <jes at sgi dot com>, Tony Luck <tony dot luck at intel dot com>, discuss at x86-64 dot org, linux-kernel at vger dot kernel dot org, libc-alpha at sourceware dot org, vojtech at suse dot cz, linux-ia64 at vger dot kernel dot org
- Date: Fri, 16 Jun 2006 18:24:52 +0200
- Subject: Re: FOR REVIEW: New x86-64 vsyscall vgetcpu()
- References: <200606140942.31150.ak@suse.de> <200606161737.06132.ak@suse.de> <20060616155804.GN3823@sunsite.mff.cuni.cz>
On Friday 16 June 2006 17:58, Jakub Jelinek wrote:
> On Fri, Jun 16, 2006 at 05:37:06PM +0200, Andi Kleen wrote:
> > On Friday 16 June 2006 17:31, Zoltan Menyhart wrote:
> > > Andi Kleen wrote:
> > >
> > > > That is not how user space TLS works. It usually has a base a register.
> > >
> > > Can you please give me a real life (simplified) example?
> >
> > On x86-64 it's just %fs:offset. gcc is a bit dumb on this and usually
> > loads the base address from %fs:0 first.
>
> GCC is not dumb, unless you force it with -mno-tls-direct-seg-refs.
> Guess you are bitten by SUSE GCC hack which makes -mno-tls-direct-seg-refs
> the default (especially on x86-64 it is a really bad idea).
I apparently got indeed.
I wonder why it happened on x86-64 though - i thought there were no negative
offsets on x86-64 TLS.
-Andi