Re: PML (Page Modification Logging) design for Xen

[Kai Huang <kai.huang@linux.intel.com>]

On 02/12/2015 08:42 PM, Tim Deegan wrote:
> At 07:08 +0000 on 12 Feb (1423721283), Tian, Kevin wrote:
>> for general log dirty, ept_invalidate_emt is required because there is
>> access permission change (dirtied page becomes rw after 1st fault,
>> so need to change them back to ro again for the new dirty tracking
>> round). But for PML, there's no permission change at all (always rw),
>> so such behavior should be noted by general logdirty layer for better
>> optimization.
> AIUI the reason for calling ept_invalidate_emt() is to avoid having to
> update a large number of EPTEs at once.  If you still need to update a
> large number of EPTEs (to clear the Dirty bits), that has to me
> preemptable, or else use ept_invalidate_emt().
>
> Or have I misunderstood?
I think you are correct. We still need to use ept_invalidate_emt for 
clearing D-bit, unless we invent a new paging layer interface, say 
paging_enable_log_dirty_gfn, which explicitly enables log-dirty for 
single GFN, either by write protection, or clearing D-bit, in case of PML.

Thanks,
-Kai
>
> Tim.