Re: [PATCH 2/2] KVM: x86/mmu: Don't step down in the TDP iterator when zapping all SPTEs

[Paolo Bonzini]

On 12/08/21 19:46, Sean Christopherson wrote:
> > > 	if (iter->level == iter->min_level)
> > > 		return false;
> > >
> > > 	/*
> > > 	 * Reread the SPTE before stepping down to avoid traversing into page
> > > 	 * tables that are no longer linked from this entry.
> > > 	 */
> > > 	iter->old_spte = READ_ONCE(*rcu_dereference(iter->sptep));  \
> > >                                                                        ---> this is the code that is avoided
> > > 	child_pt = spte_to_child_pt(iter->old_spte, iter->level);   /
> > > 	if (!child_pt)
> > > 		return false;
> > Ah, right - so I agree with Ben that it's not too important.
> Ya.  There is a measurable performance improvement, but it's really only
> meaningful when there aren't many SPTEs to zap, otherwise the cost of zapping
> completely dominates the time.

I don't understand.  When try_step_down is called by tdp_iter_next, all 
it does is really just the READ_ONCE, because spte_to_child_pt will see 
a non-present PTE and return immediately.  Why do two, presumably cache 
hot, reads cause a measurable performance improvement?

Paolo