From mboxrd@z Thu Jan 1 00:00:00 1970 From: Yang Zhang Subject: Re: [PATCH 1/2] KVM: nVMX: fix msr bitmaps to prevent L2 from accessing L0 x2APIC Date: Wed, 10 Aug 2016 13:55:21 +0800 Message-ID: References: <20160808181623.12132-1-rkrcmar@redhat.com> <20160808181623.12132-2-rkrcmar@redhat.com> <20160809122319.GA2607@potion> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Jim Mattson , Wincy Van , Paolo Bonzini , Bandan Das To: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= Return-path: In-Reply-To: <20160809122319.GA2607@potion> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 2016/8/9 20:23, Radim Krčmář wrote: > 2016-08-09 17:32+0800, Yang Zhang: >> On 2016/8/9 2:16, Radim Krčmář wrote: >>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c >>> @@ -6995,16 +6982,21 @@ static int handle_vmon(struct kvm_vcpu *vcpu) >>> return 1; >>> } >>> >>> + if (cpu_has_vmx_msr_bitmap()) { >>> + vmx->nested.msr_bitmap = >>> + (unsigned long *)__get_free_page(GFP_KERNEL); >>> + if (!vmx->nested.msr_bitmap) >>> + goto out_msr_bitmap; >>> + } >>> + >> >> We export msr_bitmap to guest even it is not supported by hardware. So we >> need to allocate msr_bitmap for L1 unconditionally. > > We do emulate the feature, but the vmx->nested.msr_bitmap is used only > if VMX supports it to avoid some VM exits: > >>> @@ -9957,10 +9938,10 @@ static void prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12) >>> } >>> >>> if (cpu_has_vmx_msr_bitmap() && >>> - exec_control & CPU_BASED_USE_MSR_BITMAPS) { >>> - nested_vmx_merge_msr_bitmap(vcpu, vmcs12); >>> - /* MSR_BITMAP will be set by following vmx_set_efer. */ >>> - } else >>> + exec_control & CPU_BASED_USE_MSR_BITMAPS && >>> + nested_vmx_merge_msr_bitmap(vcpu, vmcs12)) >>> + ; /* MSR_BITMAP will be set by following vmx_set_efer. */ >>> + else >>> exec_control &= ~CPU_BASED_USE_MSR_BITMAPS; > > The else branch is taken if !cpu_has_vmx_msr_bitmap() and disables msr > bitmaps. Similar check for vmx_set_msr_bitmap(), so the NULL doesn't > even get written to VMCS. > > KVM always uses L1's msr bitmaps when emulating the feature. > Yes, you are right. I forget the bitmap only used by hardware.:( -- Yang Alibaba Cloud Computing