Re: [PATCH RFC 15/19] iommu/arm-smmu-v3: Add set_dirty_tracking_range() support

Jason Gunthorpe
2022-04-29T16:11:42Z

On Fri, Apr 29, 2022 at 03:45:23PM +0100, Joao Martins wrote:
> On 4/29/22 13:23, Jason Gunthorpe wrote:
> > 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.
> >
> Hmm, OK. But aren't really picking a poison here? On ARM it's the difference
> from switching the setting the DBM bit and put the IOPTE as writeable-clean (which
> is clearing another bit) versus read-and-clear-when-dirty-track-start which means
> we need to re-walk the pagetables to clear one bit.

Yes, I don't think a iopte walk is avoidable?

> It's walking over ranges regardless.

Also, keep in mind start should always come up in a 0 dirties state on
all platforms. So all implementations need to do something to wipe the
dirty state, either explicitly during start or restoring all clean
during stop.

A common use model might be to just destroy the iommu_domain without
doing stop so prefering the clearing io page table at stop might be a
better overall design.

Jason