Re: [crash, bisected] Re: [PATCH 3/4] x86_64: Fold pda into per cpuarea

[Jeremy Fitzhardinge]

Christoph Lameter wrote:
> On Fri, 20 Jun 2008, Jeremy Fitzhardinge wrote:
>
>   
> > > The loader setup for the percpu section changes with zero basing. Maybe that
> > > has bad side effects
> > >       
> > How does it work?  The symbols in the percpu segment are 0-based, but where
> > does the data for the sections which correspond to that segment go?
> >     
>
> Its loaded at __per_cpu_load but the symbols have addresses starting at 0.
>   

Yes, which leads to an odd-looking ELF file where the Phdrs aren't 
sorted by virtual address order.  I'm wondering what would happen if a 
bootloader that actually understood ELF files tried to load it as an 
actual ELF file...

> > So the question is what kernel virtual address is it being loaded to?
> > __per_cpu_load is ffffffff808d1000, so ffffffff808d6000 is what you'd
> > expect...
> >     
>
> Correct.
>   

Well, reading back from that address got zeros, so something is amiss.

> > Hm, but what happens when this gets converted to bzImage?  Hm, looks OK, I
> > think.
> >
> > BTW, I think __per_cpu_load will cause trouble if you make a relocatable
> > kernel, being an absolute symbol.  But I have relocation off at the moment.
> >     
>
> Hmmm.... we could add the relocation offset to __per_cpu_load? 
> __per_cpu_load is used very sparingly. Basically only useful during early 
> boot and when a new per cpu area has to be setup. In that case we want to 
> copy from __per_cpu_load to the newly allocated percpu area.
>   

Yes, it should be fairly easy to manually relocate it by applying the 
(load - link) offset to it.


   J
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