From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757578Ab0IZNJP (ORCPT ); Sun, 26 Sep 2010 09:09:15 -0400 Received: from mx1.redhat.com ([209.132.183.28]:37180 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752496Ab0IZNJO (ORCPT ); Sun, 26 Sep 2010 09:09:14 -0400 Message-ID: <4C9F45F3.7000405@redhat.com> Date: Sun, 26 Sep 2010 15:09:07 +0200 From: Avi Kivity User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.9) Gecko/20100907 Fedora/3.1.3-1.fc13 Lightning/1.0b3pre Thunderbird/3.1.3 MIME-Version: 1.0 To: Xiao Guangrong CC: Marcelo Tosatti , LKML , KVM Subject: Re: [PATCH 4/4] KVM: MMU: Don't touch unsync sp in kvm_mmu_pte_write() References: <4C976D48.6020400@cn.fujitsu.com> <4C976DF6.1020905@cn.fujitsu.com> <4C977CBF.6030702@redhat.com> <4C9AC283.3000307@cn.fujitsu.com> In-Reply-To: <4C9AC283.3000307@cn.fujitsu.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/23/2010 04:59 AM, Xiao Guangrong wrote: > On 09/20/2010 11:24 PM, Avi Kivity wrote: > > On 09/20/2010 04:21 PM, Xiao Guangrong wrote: > >> Gfn may have many shadow pages, when one sp need be synced, we write > >> protected sp->gfn and sync this sp but we keep other shadow pages > >> asynchronous > >> > >> So, while gfn happen page fault, let it not touches unsync page, the > >> unsync > >> page only updated at invlpg/flush TLB time > >> > >> @@ -3157,6 +3164,9 @@ void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, > >> gpa_t gpa, > >> > >> mask.cr0_wp = mask.cr4_pae = mask.nxe = 1; > >> for_each_gfn_indirect_valid_sp(vcpu->kvm, sp, gfn, node) { > >> + if (sp->unsync) > >> + continue; > >> + > >> > > > > Not sure this is a win. If a gpte is updated from having p=0 to p=1 (or > > permissions upgraded), we may not have an invlpg to sync the spte, since > > the hardware doesn't require it. With this change, we may get an extra > > #PF. > > > > Avi, > > Thanks for your review, i think this case is not too bad since: > > 1: This case only impacts local vcpu since if permissions is increased, it's > no need send IPT to flush remote vcpu's tlb, so even if we update unsync > spte in kvm_mmu_pte_write() path, the #PF still occur on other vcpus. IIRC, the cpu will re-validate the tlb entry from the page tables before issuing a fault, so we won't see a spurious fault. Not 100% sure. For !P -> P, there won't be a tlb entry, so 100% there won't be a spurious fault. > 2: If the unsync sp which is updated in kvm_mmu_pte_write() is not using by the > vcpu, it will sync automatically after it's loaded. True, and this is a likely case. > 3: If the sp is using, update this sp in kvm_mmu_pte_write() will avoid extra #PF, > in this case, two(or more) sps have the same gfn, there are mapped in the same > page table and with different kinds(unsync/sync), i thinks this case is infrequency. > And even we updated it, we can not sure it can be accessed latter, If it's infrequent, the why do we optimize it? > So, i think it's better lazily update unsync sp until it's used or the flush time, > your opinion? :-) > Any performance numbers? To me it seems saving a possible exit is worth extra computation. -- error compiling committee.c: too many arguments to function