Re: FOR REVIEW: New x86-64 vsyscall vgetcpu()

Andi Kleen
2006-06-16T16:24:52Z

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