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

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Date:   Thu, 12 Aug 2021 17:33:30 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Ben Gardon <bgardon@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] KVM: x86/mmu: Don't step down in the TDP iterator
 when zapping all SPTEs

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;


My comment wasn't all that accurate either.  Maybe this?

	/*
	 * No need to try to step down in the iterator when zapping all SPTEs,
	 * zapping the top-level non-leaf SPTEs will recurse on their children.
	 */
	 int min_level = zap_all ? root->role.level : PG_LEVEL_4K;

> 	 */
> 	int min_level = zap_all ? root->role.level : PG_LEVEL_4K;
> 
> Paolo
>