KVM: nVMX: Switch to vmcs01 to update APIC page on-demand if L2 is active

If the KVM-owned APIC-access page is migrated while L2 is running,
temporarily load vmcs01 and immediately update APIC_ACCESS_ADDR instead
of deferring the update until the next nested VM-Exit.  Once changing
the virtual APIC mode is converted to always do on-demand updates, all
of the "defer until vmcs01 is active" logic will be gone.

Reviewed-by: Chao Gao <chao.gao@intel.com>
Link: https://patch.msgid.link/20260109034532.1012993-7-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/vmx/nested.c

@@ -5129,11 +5129,6 @@ void __nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
 
 	nested_put_vmcs12_pages(vcpu);
 
-	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 ((vm_exit_reason != -1) &&
 	    (enable_shadow_vmcs || nested_vmx_is_evmptr12_valid(vmx)))
 		vmx->nested.need_vmcs12_to_shadow_sync = true;

arch/x86/kvm/vmx/vmx.c

@@ -6904,11 +6904,8 @@ void vmx_set_apic_access_page_addr(struct kvm_vcpu *vcpu)
 	kvm_pfn_t pfn;
 	bool writable;
 
-	/* Defer reload until vmcs01 is the current VMCS. */
-	if (is_guest_mode(vcpu)) {
-		to_vmx(vcpu)->nested.reload_vmcs01_apic_access_page = true;
-		return;
-	}
+	/* Note, the VIRTUALIZE_APIC_ACCESSES check needs to query vmcs01. */
+	guard(vmx_vmcs01)(vcpu);
 
 	if (!(secondary_exec_controls_get(to_vmx(vcpu)) &
 	    SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES))

arch/x86/kvm/vmx/vmx.h

@@ -132,7 +132,6 @@ 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