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SubjectRe: i386/x86_64 segment register issuses (Re: PATCH: Fix x86 segment register access)


On Tue, 29 Mar 2005, H. J. Lu wrote:
>
> > the smaller and faster version do not want to just rely on gas
> > automatically getting it right, especially since gas has historically been
> > very very bad at getting things right.
>
> We are fixing those issues in assembler. If people run into problems
> like that with gas, they can report them. They will be fixed.

It's fine if gas fixes things. It's not fine if gas breaks things that
used to work, for no really good reason.

> > What is the advantage of not allowing "movl %ds,mem"? Really? Especially
> > since I suspect the kernel is pretty much the only one who does this, and
> > the kernel really does do it on purpose. The kernel explicitly wants the
> > 32-bit version, knowing that the upper bits are undefined.
> >
>
> Kernel has
>
> unsigned gsindex;
> asm volatile("movl %%gs,%0" : "=g" (gsindex));

Ok, that's a real x86-64 bug, it seems. Andi, please fix, preferably by
just making the "g" be a "r".

However, your argument isn't very valid, since:

> The new assembler will make sure that it won't happen.

Not true, since the suggestion was just to change all segment "movl"
things to "mov", at which point the same old bug is still there, and the
assembler didn't really help us at all.
See the problem? You're not actually protecting anything. The change just
makes it _harder_ to make sizes explicit, and suddenly we have to trust an
assembler to be clever about sizes, when that assembler historically has
definitely _not_ been very clever about them at all.

Linus
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