Re: [PATCH] KVM: x86/mmu: Skip !MMU-present SPTEs when removing SP in exclusive mode

[Paolo Bonzini]

On 10/03/21 23:24, Ben Gardon wrote:
> On Wed, Mar 10, 2021 at 1:14 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> > On Wed, Mar 10, 2021, Paolo Bonzini wrote:
> > > On 10/03/21 01:30, Sean Christopherson wrote:
> > > > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > > > index 50ef757c5586..f0c99fa04ef2 100644
> > > > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > > > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > > > @@ -323,7 +323,18 @@ static void handle_removed_tdp_mmu_page(struct kvm *kvm, u64 *pt,
> > > >                              cpu_relax();
> > > >                      }
> > > >              } else {
> > > > +                   /*
> > > > +                    * If the SPTE is not MMU-present, there is no backing
> > > > +                    * page associated with the SPTE and so no side effects
> > > > +                    * that need to be recorded, and exclusive ownership of
> > > > +                    * mmu_lock ensures the SPTE can't be made present.
> > > > +                    * Note, zapping MMIO SPTEs is also unnecessary as they
> > > > +                    * are guarded by the memslots generation, not by being
> > > > +                    * unreachable.
> > > > +                    */
> > > >                      old_child_spte = READ_ONCE(*sptep);
> > > > +                   if (!is_shadow_present_pte(old_child_spte))
> > > > +                           continue;
> > > >                      /*
> > > >                       * Marking the SPTE as a removed SPTE is not
>
> This optimization also makes me think we could also skip the
> __handle_changed_spte call in the read mode case if the SPTE change
> was !PRESENT -> REMOVED.
Yes, I think so.  It should be a separate patch anyway, so I've queued 
this one.

Paolo