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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: Fri, 13 Aug 2021 16:13:22 +0000	[thread overview]
Message-ID: <YRaaIi9Go38E3mUh@google.com> (raw)
In-Reply-To: <b08a7751-20c3-26fc-522e-c4cf274d9a6c@redhat.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.

  reply	other threads:[~2021-08-13 16:13 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
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 [this message]
2021-08-13 16:38                   ` Paolo Bonzini

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