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

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Message-Id: <200711291017.52727.rusty@rustcorp.com.au>
Date:	Thu, 29 Nov 2007 10:17:52 +1100
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Christoph Lameter <clameter@....com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Andi Kleen <ak@...e.de>, Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: [patch 05/14] percpu: Use a Kconfig variable to configure arch specific percpu setup

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.
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