KVM: nVMX: Preset *DT exiting in vmcs02 when emulating UMIP

KVM dynamically toggles SECONDARY_EXEC_DESC to intercept (a subset of)
instructions that are subject to User-Mode Instruction Prevention, i.e.
VMCS.SECONDARY_EXEC_DESC == CR4.UMIP when emulating UMIP.  Preset the
VMCS control when preparing vmcs02 to avoid unnecessarily VMWRITEs,
e.g. KVM will clear VMCS.SECONDARY_EXEC_DESC in prepare_vmcs02_early()
and then set it in vmx_set_cr4().
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -2085,6 +2085,14 @@ static void prepare_vmcs02_early(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12)
 		/* VMCS shadowing for L2 is emulated for now */
 		exec_control &= ~SECONDARY_EXEC_SHADOW_VMCS;
 
+		/*
+		 * Preset *DT exiting when emulating UMIP, so that vmx_set_cr4()
+		 * will not have to rewrite the controls just for this bit.
+		 */
+		if (!boot_cpu_has(X86_FEATURE_UMIP) && vmx_umip_emulated() &&
+		    (vmcs12->guest_cr4 & X86_CR4_UMIP))
+			exec_control |= SECONDARY_EXEC_DESC;
+
 		if (exec_control & SECONDARY_EXEC_VIRTUAL_INTR_DELIVERY)
 			vmcs_write16(GUEST_INTR_STATUS,
 				vmcs12->guest_intr_status);