Re: [RFC PATCH 0/4] KVM: x86/mmu: Rework marking folios dirty/accessed

[Sean Christopherson <seanjc@google.com>]

On Thu, Apr 04, 2024, David Hildenbrand wrote:
> On 04.04.24 00:19, Sean Christopherson wrote:
> > Hmm, we essentially already have an mmu_notifier today, since secondary MMUs need
> > to be invalidated before consuming dirty status.  Isn't the end result essentially
> > a sane FOLL_TOUCH?
> 
> Likely. As stated in my first mail, FOLL_TOUCH is a bit of a mess right now.
> 
> Having something that makes sure the writable PTE/PMD is dirty (or
> alternatively sets it dirty), paired with MMU notifiers notifying on any
> mkclean would be one option that would leave handling how to handle dirtying
> of folios completely to the core. It would behave just like a CPU writing to
> the page table, which would set the pte dirty.
> 
> Of course, if frequent clearing of the dirty PTE/PMD bit would be a problem
> (like we discussed for the accessed bit), that would not be an option. But
> from what I recall, only clearing the PTE/PMD dirty bit is rather rare.

And AFAICT, all cases already invalidate secondary MMUs anyways, so if anything
it would probably be a net positive, e.g. the notification could more precisely
say that SPTEs need to be read-only, not blasted away completely.