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RE: [Xen-devel] Why is 'emulate' as good as writable PT's?

  • To: "Andrew Theurer" <habanero@xxxxxxxxxx>, "Xen development list" <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • From: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
  • Date: Mon, 5 Jun 2006 23:17:57 +0100
  • Cc: Rolf Neugebauer <rn@xxxxxxx>
  • Delivery-date: Mon, 05 Jun 2006 15:19:02 -0700
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: AcaI6ah0lkMiLFmQQW2hJkNP7Trc/QAAjQ0w
  • Thread-topic: [Xen-devel] Why is 'emulate' as good as writable PT's?

> Could there be situations were we are inadvertently triggering a
> writable page table, where we should just be doing a

Almost certainly. Singleton (or small batch) updates should not be using
writeable pagetables, and should use update_va_mapping (or mmu_update if
the VA isn't known or may not be mapped).

~18 months ago Rolf wrote and checked in profile code to collect a
histogram of the number of entries found to be modified when writeable
pagetables are flushed.
At the time there was a big spike at '1' which was fixed, but with all
the various linux version upgrades it likely needs revisiting. 

The profile code also records the EIP that caused the writeable
pagetables operation, so if you print out the value a few times you'll
quickly find the culprit.


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