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 19:33, Sean Christopherson wrote:
> On Thu, Aug 12, 2021, Paolo Bonzini wrote:
> > On 12/08/21 19:07, Sean Christopherson wrote:
> > > Yeah, I was/am on the fence too, I almost included a blurb in the cover letter
> > > saying as much.  I'll do that for v2 and let Paolo decide.
> >
> > I think it makes sense to have it.  You can even use the ternary operator
>
> Hah, yeah, I almost used a ternary op.  Honestly don't know why I didn't, guess
> my brain flipped a coin.
>
> > 	/*
> > 	 * When zapping everything, all entries at the top level
> > 	 * ultimately go away, and the levels below go down with them.
> > 	 * So do not bother iterating all the way down to the leaves.
>
> The subtle part is that try_step_down() won't actually iterate down because it
> explicitly rereads and rechecks the SPTE.
>
> 	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.

Paolo