Re: Tracking changes to writable page tables

[Jacob Gorm Hansen <jacobg@diku.dk>]

Ian Pratt wrote:
>>It looks like xenlinux 2.6 now uses writable page tables by 
>>default (I can't find the switch to enable/disable them).
>>
>>I need to track writes to page tables, and I was wondering if 
>>it is possible to get a page fault upon writes to read-only 
>>mapped page tables, as when writing to other read-only mappings?
> 
> 
> Hmm, I can see that the current model makes like difficult for you
> (though self-migration is a pretty twisted thing to be trying to do
> anyway :-)
> 
> I wander if we should steal one of the OS bits in the PTE to
> differentiate writable pagetable mappings? You could then take a normal
> RO fault, set the bit, and then retry to have Xen do pagetable the work.
> 
> This would arguably be cleaner, though we've been reluctant to steal OS
> bits just incase there's some OS out there that uses them all.

Actually, it seems this could be done without using an AVL bit, as this
is really per-frame rather than per-VA information. If the pte-pin call
could add a 'propagate page fault' flag to the PTE frame, then I could
still get the first fault to the guest, and then remove the pinning bit
to prevent the rest of the faults.

Jacob