From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-10.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8FDE9C4708A for ; Fri, 28 May 2021 00:18:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 72073613B4 for ; Fri, 28 May 2021 00:18:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234705AbhE1AUX (ORCPT ); Thu, 27 May 2021 20:20:23 -0400 Received: from out30-56.freemail.mail.aliyun.com ([115.124.30.56]:50120 "EHLO out30-56.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229911AbhE1AUU (ORCPT ); Thu, 27 May 2021 20:20:20 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R161e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e01424;MF=laijs@linux.alibaba.com;NM=1;PH=DS;RN=16;SR=0;TI=SMTPD_---0UaJXb16_1622161123; Received: from C02XQCBJJG5H.local(mailfrom:laijs@linux.alibaba.com fp:SMTPD_---0UaJXb16_1622161123) by smtp.aliyun-inc.com(127.0.0.1); Fri, 28 May 2021 08:18:44 +0800 Subject: Re: [PATCH] KVM: X86: fix tlb_flush_guest() To: Sean Christopherson , Paolo Bonzini Cc: Lai Jiangshan , linux-kernel@vger.kernel.org, Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Thomas Gleixner , Ingo Molnar , Borislav Petkov , x86@kernel.org, "H. Peter Anvin" , =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , kvm@vger.kernel.org, Maxim Levitsky References: <20210527023922.2017-1-jiangshanlai@gmail.com> <78ad9dff-9a20-c17f-cd8f-931090834133@redhat.com> From: Lai Jiangshan Message-ID: Date: Fri, 28 May 2021 08:18:43 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021/5/28 00:13, Sean Christopherson wrote: > +Maxim - A proper fix for this bug might fix your shadow paging + win10 boot > issue, this also affects the KVM_REQ_HV_TLB_FLUSH used for HyperV PV > flushing. > > On Thu, May 27, 2021, Paolo Bonzini wrote: >> On 27/05/21 04:39, Lai Jiangshan wrote: >>> From: Lai Jiangshan >>> >>> For KVM_VCPU_FLUSH_TLB used in kvm_flush_tlb_multi(), the guest expects >>> the hypervisor do the operation that equals to native_flush_tlb_global() >>> or invpcid_flush_all() in the specified guest CPU. >>> >>> When TDP is enabled, there is no problem to just flush the hardware >>> TLB of the specified guest CPU. >>> >>> But when using shadowpaging, the hypervisor should have to sync the >>> shadow pagetable at first before flushing the hardware TLB so that >>> it can truely emulate the operation of invpcid_flush_all() in guest. >> >> Can you explain why? > > KVM's unsync logic hinges on guest TLB flushes. For page permission modifications > that require a TLB flush to take effect, e.g. making a writable page read-only, > KVM waits until the guest explicitly does said flush to propagate the changes to > the shadow page tables. E.g. failure to sync PTEs could result in a read-only 4k > page being writable when the guest expects it to be read-only. > >> Also it is simpler to handle this in kvm_vcpu_flush_tlb_guest, using "if >> (tdp_enabled). This provides also a single, good place to add a comment >> with the explanation of what invalid entries KVM_REQ_RELOAD is presenting. > > Ya. > > KVM_REQ_MMU_RELOAD is overkill, nuking the shadow page tables will completely > offset the performance gains of the paravirtualized flush > > And making a request won't work without revamping the order of request handling > in vcpu_enter_guest(), e.g. KVM_REQ_MMU_RELOAD and KVM_REQ_MMU_SYNC are both > serviced before KVM_REQ_STEAL_UPDATE. Yes, it just fixes the said problem in the simplest way. I copied KVM_REQ_MMU_RELOAD from kvm_handle_invpcid(INVPCID_TYPE_ALL_INCL_GLOBAL). (If the guest is not preempted, it will call invpcid_flush_all() and will be handled by this way) The improvement code will go later, and will not be backported. The proper way to flush guest is to use code in https://lore.kernel.org/lkml/20210525213920.3340-1-jiangshanlai@gmail.com/ as: + kvm_mmu_sync_roots(vcpu); + kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu); //or just call flush_current directly + for (i = 0; i < KVM_MMU_NUM_PREV_ROOTS; i++) + vcpu->arch.mmu->prev_roots[i].need_sync = true; If need_sync patch is not accepted, we can just use kvm_mmu_sync_roots(vcpu) to keep the current pagetable and use kvm_mmu_free_roots() to free all the other roots in prev_roots. > > Cleaning up and documenting the MMU related requests is on my todo list, but the > immediate fix should be tiny and I can do my cleanups on top. > > I believe the minimal fix is: > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index 81ab3b8f22e5..b0072063f9bf 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -3072,6 +3072,9 @@ static void kvm_vcpu_flush_tlb_all(struct kvm_vcpu *vcpu) > static void kvm_vcpu_flush_tlb_guest(struct kvm_vcpu *vcpu) > { > ++vcpu->stat.tlb_flush; > + > + if (!tdp_enabled) > + kvm_mmu_sync_roots(vcpu); it doesn't handle prev_roots which are also needed as shown in kvm_handle_invpcid(INVPCID_TYPE_ALL_INCL_GLOBAL). > static_call(kvm_x86_tlb_flush_guest)(vcpu); For tdp_enabled, I think it is better to use kvm_x86_tlb_flush_current() to make it consistent with other shadowpage code. > } > >