Re: [RFC 00/15] x86_64: Optimize percpu accesses

[Arjan van de Ven]

On Wed, 09 Jul 2008 13:22:06 -0700
ebiederm@xxxxxxxxxxxx (Eric W. Biederman) wrote:

> Jeremy Fitzhardinge <jeremy@xxxxxxxx> writes:
> 
> > It's just the stack canary.  It isn't library accesses; it's the
> > code gcc generates:
> >
> > foo:	subq	$152, %rsp
> >        movq    %gs:40, %rax
> >        movq    %rax, 136(%rsp)
> > ...
> >        movq    136(%rsp), %rdx
> >        xorq    %gs:40, %rdx
> >        je      .L3
> >        call    __stack_chk_fail
> > .L3:
> >        addq    $152, %rsp
> >        .p2align 4,,4
> >        ret
> >
> >
> > There are two irritating things here:
> >
> > One is that the kernel supports -fstack-protector for x86-64, which
> > forces us into all these contortions in the first place.  We don't
> > support stack-protector for 32-bit (gcc does), and things are much
> > easier.
> 
> How does gcc know to use %gs instead of the usual %fs for accessing
> the stack protector variable?  My older gcc-4.1.x on ubuntu always
> uses %fs.

ubuntu broke gcc (they don't want to have compiler flags per package so
patches stuff in gcc instead).


> I think the unification is valid and useful, and that trying to keep
> that stupid stack canary working is currently more trouble then it is
> worth.

I think that "unification over everything" is stupid, especially if it
removes useful features.



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