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

[Andrew Theurer <habanero@xxxxxxxxxx>]

Ian Pratt wrote:
> > Could there be situations were we are inadvertently triggering a
> > writable page table, where we should just be doing a
> update_va_mapping()?
>
> 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.
Thanks!  It looks like the histogram and EIP logs in ptwr_flush are 
still there, so we'll run again with perfc=y and see if we can pinpoint 
the culprit.
-Andrew

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