From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Kiszka Subject: Re: [PATCH v2 5/8] KVM: nVMX: Fix guest CR3 read-back on VM-exit Date: Tue, 06 Aug 2013 17:48:54 +0200 Message-ID: <52011AE6.2010006@siemens.com> References: <0816baee846f9c8f4d54c6738b2582a95f9c56a3.1375778397.git.jan.kiszka@web.de> <20130806101236.GN8218@redhat.com> <20130806140248.GB8218@redhat.com> <20130806144117.GD8218@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: Gleb Natapov , Paolo Bonzini , kvm , Xiao Guangrong , "Nakajima, Jun" , Arthur Chunqi Li To: "Zhang, Yang Z" Return-path: Received: from david.siemens.de ([192.35.17.14]:22181 "EHLO david.siemens.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755113Ab3HFPtH (ORCPT ); Tue, 6 Aug 2013 11:49:07 -0400 In-Reply-To: Sender: kvm-owner@vger.kernel.org List-ID: On 2013-08-06 17:04, Zhang, Yang Z wrote: > Gleb Natapov wrote on 2013-08-06: >> On Tue, Aug 06, 2013 at 02:12:51PM +0000, Zhang, Yang Z wrote: >>> Gleb Natapov wrote on 2013-08-06: >>>> On Tue, Aug 06, 2013 at 11:44:41AM +0000, Zhang, Yang Z wrote: >>>>> Gleb Natapov wrote on 2013-08-06: >>>>>> On Tue, Aug 06, 2013 at 10:39:59AM +0200, Jan Kiszka wrote: >>>>>>> From: Jan Kiszka >>>>>>> >>>>>>> If nested EPT is enabled, the L2 guest may change CR3 without any >>>>>>> exits. We therefore have to read the current value from the VMCS >>>>>>> when switching to L1. However, if paging wasn't enabled, L0 tracks >>>>>>> L2's CR3, and GUEST_CR3 rather contains the real-mode identity map. >>>>>>> So we need to retrieve CR3 from the architectural state after >>>>>>> conditionally updating it - and this is what kvm_read_cr3 does. >>>>>>> >>>>>> I have a headache from trying to think about it already, but >>>>>> shouldn't >>>>>> L1 be the one who setups identity map for L2? I traced what >>>>>> vmcs_read64(GUEST_CR3)/kvm_read_cr3(vcpu) return here and do not >>>>>> see >>>>> Here is my understanding: >>>>> In vmx_set_cr3(), if enabled ept, it will check whether target >>>>> vcpu is enabling >>>> paging. When L2 running in real mode, then target vcpu is not >>>> enabling paging and it will use L0's identity map for L2. If you >>>> read GUEST_CR3 from VMCS, then you may get the L2's identity map >>>> not >> L1's. >>>>> >>>> Yes, but why it makes sense to use L0 identity map for L2? I didn't >>>> see different vmcs_read64(GUEST_CR3)/kvm_read_cr3(vcpu) values because >>>> L0 and L1 use the same identity map address. When I changed identity >>>> address L1 configures vmcs_read64(GUEST_CR3)/kvm_read_cr3(vcpu) are >>>> indeed different, but the real CR3 L2 uses points to L0 identity map. >>>> If I zero L1 identity map page L2 still works. >>>> >>> If L2 in real mode, then L2PA == L1PA. So L0's identity map also works >>> if L2 is in real mode. >>> >> That not the point. It may work accidentally for kvm on kvm, but what >> if other hypervisor plays different tricks and builds different ident map for its guest? > Yes, if other hypervisor doesn't build the 1:1 mapping for its guest, it will fail to work. But I cannot imagine what kind of hypervisor will do this and what the purpose is. > Anyway, current logic is definitely wrong. It should use L1's identity map instead L0's. So something like this is rather needed? diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c index 44494ed..60a3644 100644 --- a/arch/x86/kvm/vmx.c +++ b/arch/x86/kvm/vmx.c @@ -3375,8 +3375,10 @@ static void vmx_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3) if (enable_ept) { eptp = construct_eptp(cr3); vmcs_write64(EPT_POINTER, eptp); - guest_cr3 = is_paging(vcpu) ? kvm_read_cr3(vcpu) : - vcpu->kvm->arch.ept_identity_map_addr; + if (is_paging(vcpu) || is_guest_mode(vcpu)) + guest_cr3 = kvm_read_cr3(vcpu) : + else + guest_cr3 = vcpu->kvm->arch.ept_identity_map_addr; ept_load_pdptrs(vcpu); } Jan -- Siemens AG, Corporate Technology, CT RTC ITP SES-DE Corporate Competence Center Embedded Linux