linux-kernel - Re: [PATCH 5/7] Use %gs for per-cpu sections in kernel

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Date:	Tue, 26 Sep 2006 09:33:14 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Andi Kleen <ak@....de>,
	lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
	virtualization <virtualization@...ts.osdl.org>
Subject: Re: [PATCH 5/7] Use %gs for per-cpu sections in kernel

On Sun, 2006-09-24 at 23:25 -0700, Jeremy Fitzhardinge wrote:
> Rusty Russell wrote:
> > I don't think so.  There's *never* address subtraction, there's
> > sometimes 32 bit wrap (glibc uses this to effect subtraction, sure).
> > But there's no wrap here.
> >   
> Hm, I guess, so long as you assume the kernel data segment is always 
> below the kernel heap.

Agreed, we should BUG_ON() in case anyone ever changes this...  I will
create a patch for this...

> > To test, I changed the following:
> >
> > --- smpboot.c.~8~	2006-09-25 15:51:50.000000000 +1000
> > +++ smpboot.c	2006-09-25 16:00:36.000000000 +1000
> > @@ -926,8 +926,9 @@
> >  					      unsigned long per_cpu_off)
> >  {
> >  	unsigned limit, flags;
> > +	extern char __per_cpu_end[];
> >  
> > -	limit = (1 << 20);
> > +	limit = PAGE_ALIGN((long)__per_cpu_end) >> PAGE_SHIFT;
> >   
> limit is a size, rather than the end address, so this isn't quite right.

I think it's OK.  For every "%gs:var", var will be less than
__per_cpu_end.

Thanks!
Rusty.
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