[PATCH v2 4/4] KVM: nVMX: Implement Enlightened MSR Bitmap feature

[Vitaly Kuznetsov <vkuznets@redhat.com>]

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 <vkuznets@redhat.com>
---
 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 34c580b5dbab..b36004503f75 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;
 
 	/* 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_changed && evmcs &&
+	    evmcs->hv_enlightenments_control.msr_bitmap &&
+	    evmcs->hv_clean_fields & HV_VMX_ENLIGHTENED_CLEAN_FIELD_MSR_BITMAP)
+		goto out_clear_msr_bitmap_changed;
+
 	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_changed:
 	vmx->nested.msr_bitmap_changed = false;
 
 	return true;
-- 
2.31.1