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From: Sean Christopherson <seanjc@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Ben Gardon <bgardon@google.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	kvm <kvm@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/2] KVM: x86/mmu: Don't step down in the TDP iterator when zapping all SPTEs
Date: Thu, 12 Aug 2021 17:33:30 +0000	[thread overview]
Message-ID: <YRVbamoQhvPmrEgK@google.com> (raw)
In-Reply-To: <928be04d-e60e-924c-1f3a-cb5fef8b0042@redhat.com>

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
> 

  reply	other threads:[~2021-08-12 17:33 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-12  5:07 [PATCH 0/2] KVM: x86/mmu: Fix a TDP MMU leak and optimize zap all Sean Christopherson
2021-08-12  5:07 ` [PATCH 1/2] KVM: x86/mmu: Don't skip non-leaf SPTEs when zapping all SPTEs Sean Christopherson
2021-08-12 16:18   ` Paolo Bonzini
2021-08-12 16:43     ` Sean Christopherson
2021-08-12 18:09       ` Sean Christopherson
2021-08-12  5:07 ` [PATCH 2/2] KVM: x86/mmu: Don't step down in the TDP iterator " Sean Christopherson
2021-08-12 16:47   ` Ben Gardon
2021-08-12 17:07     ` Sean Christopherson
2021-08-12 17:15       ` Paolo Bonzini
2021-08-12 17:33         ` Sean Christopherson [this message]
2021-08-12 17:38           ` Paolo Bonzini
2021-08-12 17:46             ` Sean Christopherson
2021-08-13  7:27               ` Paolo Bonzini
2021-08-13 16:13                 ` Sean Christopherson
2021-08-13 16:38                   ` Paolo Bonzini

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