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SubjectRe: [crash, bisected] Re: [PATCH 3/4] x86_64: Fold pda into per cpu area
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.


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