[PATCH v2] x86: Add a list for custom page fault handlers.

[Pekka Paalanen]

> could you please send us a patch for the whole mmiotrace kernel-side 
> feature, so that we can have a look at the general structure of this? 
> (and the interaction with change_page_attr(), etc.) Even if it's not 
> functional (and wont even build/boot) at the moment. Thanks,

Very well, first the revised custom page fault handler patch.
Changes since the previous submit:
- use spin_lock_irqsave instead of spin_lock
- Harvey Harrison's clean-up with the #ifdefs
- handler call site moved earlier
- remove sync RCU call

I'm not aware of any functional problems with this one.

Arjan, you said you don't like this. May I ask why?
This is most convinient for mmiotrace as it is meant to be a module.
I'm also using this as an excuse to let other people to get into the
page fault handler with their out-of-tree-today modules.