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[35.185.214.157]) by smtp.gmail.com with ESMTPSA id x81sm2720481pfc.22.2021.08.13.09.13.28 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 13 Aug 2021 09:13:28 -0700 (PDT) Date: Fri, 13 Aug 2021 16:13:22 +0000 From: Sean Christopherson To: Paolo Bonzini Cc: Ben Gardon , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , kvm , LKML Subject: Re: [PATCH 2/2] KVM: x86/mmu: Don't step down in the TDP iterator when zapping all SPTEs Message-ID: References: <20210812050717.3176478-1-seanjc@google.com> <20210812050717.3176478-3-seanjc@google.com> <928be04d-e60e-924c-1f3a-cb5fef8b0042@redhat.com> <7a95b2f6-a7ad-5101-baa5-6a19194695a3@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org 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.