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

[Sean Christopherson <seanjc@google.com>]

On Fri, Aug 13, 2021, Paolo Bonzini wrote:
> 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?

It's entirely possible my measurements were bad and/or noisy.  Ah, and my kernel
was running with CONFIG_PROVE_RCU=y, which makes the rcu_dereference() quite a bit
more expensive.