Re: [PATCH 10/18] KVM: Move x86's MMU notifier memslot walkers to generic code

[Paolo Bonzini]

On 31/03/21 18:20, Sean Christopherson wrote:
> Every call
> to .change_pte() is bookended by .invalidate_range_{start,end}(), i.e. the above
> missing kvm->mmu_notifier_seq++ is benign because kvm->mmu_notifier_count is
> guaranteed to be non-zero.

In fact ARM even relies on invalidate wrapping the change_pte handler.

        /*
         * The MMU notifiers will have unmapped a huge PMD before calling
         * ->change_pte() (which in turn calls kvm_set_spte_hva()) and
         * therefore we never need to clear out a huge PMD through this
         * calling path and a memcache is not required.
         */

> Assuming all of the above is correct, I'm very tempted to rip out .change_pte()
> entirely.

There is still the performance benefit from immediately remapping the 
page to the new destination without waiting for a fault.  Yes it's 
hypothetical but I would prefer to leave that change for later.

The fact that the count is nonzero means that you will not even have to 
complicate kvm_mmu_notifier_change_pte to handle the removal of 
mmu_notifier_seq; just add a patch before this one to WARN if it is 
zero.  (The rest of my review to patch 16 still holds).

Paolo

> It's been dead weight for 8+ years and no one has complained about
> KSM+KVM performance (I'd also be curious to know how much performance was gained
> by shaving VM-Exits).  As KVM is the only user of .change_pte(), dropping it in
> KVM would mean the entire MMU notifier could also go away.