On Fri, Apr 29, 2022 at 01:06:06PM +0100, Joao Martins wrote:

> > TBH I'd be inclined to just enable DBM unconditionally in 
> > arm_smmu_domain_finalise() if the SMMU supports it. Trying to toggle it 
> > dynamically (especially on a live domain) seems more trouble that it's 
> > worth.
> 
> Hmmm, but then it would strip userland/VMM from any sort of control (contrary
> to what we can do on the CPU/KVM side). e.g. the first time you do
> GET_DIRTY_IOVA it would return all dirtied IOVAs since the beginning
> of guest time, as opposed to those only after you enabled dirty-tracking.

It just means that on SMMU the start tracking op clears all the dirty
bits.

I also suppose you'd also want to install the IOPTEs as dirty to
avoid a performance regression writing out new dirties for cases where
we don't dirty track? And then the start tracking op will switch this
so map creates non-dirty IOPTEs?

Jason
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