KVM: nVMX: Reload APIC access page on nested VM-Exit only if necessary

Defer reloading L1's APIC page by logging the need for a reload and
processing it during nested VM-Exit instead of unconditionally reloading
the APIC page on nested VM-Exit.  This eliminates a TLB flush on the
majority of VM-Exits as the APIC page rarely needs to be reloaded.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200320212833.3507-28-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -4346,11 +4346,10 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 exit_reason,
 	kvm_vcpu_unmap(vcpu, &vmx->nested.pi_desc_map, true);
 	vmx->nested.pi_desc = NULL;
 
-	/*
-	 * We are now running in L2, mmu_notifier will force to reload the
-	 * page's hpa for L2 vmcs. Need to reload it for L1 before entering L1.
-	 */
+	if (vmx->nested.reload_vmcs01_apic_access_page) {
+		vmx->nested.reload_vmcs01_apic_access_page = false;
 		kvm_make_request(KVM_REQ_APIC_PAGE_RELOAD, vcpu);
+	}
 
 	if ((exit_reason != -1) && (enable_shadow_vmcs || vmx->nested.hv_evmcs))
 		vmx->nested.need_vmcs12_to_shadow_sync = true;

arch/x86/kvm/vmx/vmx.c

@@ -6142,10 +6142,14 @@ void vmx_set_virtual_apic_mode(struct kvm_vcpu *vcpu)
 
 static void vmx_set_apic_access_page_addr(struct kvm_vcpu *vcpu, hpa_t hpa)
 {
-	if (!is_guest_mode(vcpu)) {
+	/* Defer reload until vmcs01 is the current VMCS. */
+	if (is_guest_mode(vcpu)) {
+		to_vmx(vcpu)->nested.reload_vmcs01_apic_access_page = true;
+		return;
+	}
+
 	vmcs_write64(APIC_ACCESS_ADDR, hpa);
 	vmx_flush_tlb_current(vcpu);
-	}
 }
 
 static void vmx_hwapic_isr_update(struct kvm_vcpu *vcpu, int max_isr)

arch/x86/kvm/vmx/vmx.h

@@ -136,6 +136,7 @@ struct nested_vmx {
 	bool vmcs02_initialized;
 
 	bool change_vmcs01_virtual_apic_mode;
+	bool reload_vmcs01_apic_access_page;
 
 	/*
 	 * Enlightened VMCS has been enabled. It does not mean that L1 has to