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Re: [Xen-devel] Tracking changes to writable page tables

  • To: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • From: Jacob Gorm Hansen <jacobg@xxxxxxx>
  • Date: Sun, 15 May 2005 12:23:08 -0700
  • Delivery-date: Sun, 15 May 2005 19:22:51 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>

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.


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