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[35.185.214.157]) by smtp.gmail.com with ESMTPSA id g22sm3510733pfj.82.2021.11.04.18.09.57 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 04 Nov 2021 18:09:57 -0700 (PDT) Date: Fri, 5 Nov 2021 01:09:53 +0000 From: Sean Christopherson To: Vitaly Kuznetsov Cc: kvm@vger.kernel.org, Paolo Bonzini , Wanpeng Li , Jim Mattson , Maxim Levitsky , linux-kernel@vger.kernel.org Subject: Re: [PATCH v3 4/4] KVM: nVMX: Implement Enlightened MSR Bitmap feature Message-ID: References: <20211013142258.1738415-1-vkuznets@redhat.com> <20211013142258.1738415-5-vkuznets@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20211013142258.1738415-5-vkuznets@redhat.com> Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Wed, Oct 13, 2021, Vitaly Kuznetsov wrote: > Updating MSR bitmap for L2 is not cheap and rearly needed. TLFS for Hyper-V > offers 'Enlightened MSR Bitmap' feature which allows L1 hypervisor to > inform L0 when it changes MSR bitmap, this eliminates the need to examine > L1's MSR bitmap for L2 every time when 'real' MSR bitmap for L2 gets > constructed. > > Use 'vmx->nested.msr_bitmap_changed' flag to implement the feature. > > Note, KVM already uses 'Enlightened MSR bitmap' feature when it runs as a > nested hypervisor on top of Hyper-V. The newly introduced feature is going > to be used by Hyper-V guests on KVM. > > When the feature is enabled for Win10+WSL2, it shaves off around 700 CPU > cycles from a nested vmexit cost (tight cpuid loop test). > > Signed-off-by: Vitaly Kuznetsov > --- > arch/x86/kvm/hyperv.c | 2 ++ > arch/x86/kvm/vmx/nested.c | 20 ++++++++++++++++++-- > 2 files changed, 20 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c > index 6f11cda2bfa4..a00de1dbec57 100644 > --- a/arch/x86/kvm/hyperv.c > +++ b/arch/x86/kvm/hyperv.c > @@ -2516,6 +2516,8 @@ int kvm_get_hv_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid2 *cpuid, > > case HYPERV_CPUID_NESTED_FEATURES: > ent->eax = evmcs_ver; > + if (evmcs_ver) > + ent->eax |= HV_X64_NESTED_MSR_BITMAP; > > break; > > diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c > index bf4fa63ed371..7cd0c20d4557 100644 > --- a/arch/x86/kvm/vmx/nested.c > +++ b/arch/x86/kvm/vmx/nested.c > @@ -608,15 +608,30 @@ static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu, > struct vmcs12 *vmcs12) > { > int msr; > + struct vcpu_vmx *vmx = to_vmx(vcpu); > unsigned long *msr_bitmap_l1; > - unsigned long *msr_bitmap_l0 = to_vmx(vcpu)->nested.vmcs02.msr_bitmap; > - struct kvm_host_map *map = &to_vmx(vcpu)->nested.msr_bitmap_map; > + unsigned long *msr_bitmap_l0 = vmx->nested.vmcs02.msr_bitmap; > + struct hv_enlightened_vmcs *evmcs = vmx->nested.hv_evmcs; > + struct kvm_host_map *map = &vmx->nested.msr_bitmap_map; That reminds me, can my nested bitmap fixes get merged? Superficial conflicts, but still conflicts that I'd rather not have to resolve :-) https://lkml.kernel.org/r/20210924204907.1111817-1-seanjc@google.com > > /* Nothing to do if the MSR bitmap is not in use. */ > if (!cpu_has_vmx_msr_bitmap() || > !nested_cpu_has(vmcs12, CPU_BASED_USE_MSR_BITMAPS)) > return false; > > + /* > + * MSR bitmap update can be skipped when: > + * - MSR bitmap for L1 hasn't changed. > + * - Nested hypervisor (L1) is attempting to launch the same L2 as > + * before. > + * - Nested hypervisor (L1) has enabled 'Enlightened MSR Bitmap' feature > + * and tells KVM (L0) there were no changes in MSR bitmap for L2. > + */ > + if (!vmx->nested.msr_bitmap_force_recalc && evmcs && > + evmcs->hv_enlightenments_control.msr_bitmap && > + evmcs->hv_clean_fields & HV_VMX_ENLIGHTENED_CLEAN_FIELD_MSR_BITMAP) > + goto out_clear_msr_bitmap_force_recalc; Huh? Why clear it, it's already clear. Any reason not to simply return true? > + > if (kvm_vcpu_map(vcpu, gpa_to_gfn(vmcs12->msr_bitmap), map)) > return false; > > @@ -700,6 +715,7 @@ static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu, > > kvm_vcpu_unmap(vcpu, &to_vmx(vcpu)->nested.msr_bitmap_map, false); > > +out_clear_msr_bitmap_force_recalc: > vmx->nested.msr_bitmap_force_recalc = false; > > return true; > -- > 2.31.1 >