Re: [PATCH 3/3] x86_64: Use relative per-cpu offsets

[Peter Zijlstra]

On Sat, Nov 13, 2021 at 11:54:19PM -0500, Brian Gerst wrote:
> On Sat, Nov 13, 2021 at 8:18 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> >
> >
> >
> > On Sat, Nov 13, 2021, at 4:40 AM, Brian Gerst wrote:
> > > The per-cpu section is currently linked at virtual address 0, because
> > > older compilers hardcoded the stack protector canary value at a fixed
> > > offset from the start of the GS segment.  Use a standard relative offset
> > > as the GS base when the stack protector is disabled, or a newer compiler
> > > is used that supports a configurable location for the stack canary.
> >
> > Can you explain the benefit?  Also, I think we should consider dropping support for the fixed model like we did on x86_32.
> 
> This patch probably makes more sense if we drop the fixed model, as
> that gets rid of alot of code that works around having to link the
> percpu section differently.

Can someone spell out these benefits please? To me having per-cpu start
at 0 makes perfect sense, how does not having that make things better?