Re: [patch 05/14] percpu: Use a Kconfig variable to configure arch specific percpu setup

Rusty Russell
2007-11-28T23:18:17Z

On Thursday 29 November 2007 05:51:29 Christoph Lameter wrote:
> On Wed, 28 Nov 2007, Rusty Russell wrote:
> > On Wednesday 28 November 2007 05:14:47 Christoph Lameter wrote:
> > > On Tue, 27 Nov 2007, Rusty Russell wrote:
> > > > Have you considered moving x86-64's setup_per_cpu_areas into generic
> > > > code? It's a bit messier because some archs might not have set up
> > > > NUMA stuff yet, but it's logically generic...
> > >
> > > Yes that will happen later. This is just the early cleanup work. I
> > > plan to generally bring the two x86 arches in line. The pda will be
> > > folded into the per cpu area and after that its easy to do.
> >
> > Unfortunately, we tried to get rid of the x86-64 pda (like i386) but you
> > lose the ability to use the stack protection config option. That's
> > because it assumes that gs:0x68 (or something) is the stack canary; we
> > need a YA gcc change to make this gs:__builtin_stack_canary_off (where
> > gcc can emit __builtin_stack_canary_off as a weak absolute symbol, so we
> > can override it for the kernel.
>
> This works if you rebase the per cpu area at zero. gs:0x68 is still the
> stack canary.
>
> The i386 method does not work because the segment register does not
> directly point to the pda.

But the PDA itself is silly (Jeremy ported it to i386 and I balked). We have
a generic one: it's called the per-cpu data. Having a completely separate
per-cpu structure for x86-64 is a mistake.

Setting up gs as the per-cpu offset has lovely properties and avoids YA
arch-specific concept; see the i386 code. Introducing a generic
read_percpu()/write_percpu() would even make it optimal.

Cheers,
Rusty.