KVM: Load real mode segments correctly

Real mode segments to not reference the GDT or LDT; they simply compute
base = selector * 16.

Signed-off-by: Avi Kivity <avi@qumranet.com>

arch/x86/kvm/x86.c

@@ -3588,11 +3588,33 @@ static int load_segment_descriptor_to_kvm_desct(struct kvm_vcpu *vcpu,
 	return 0;
 }
 
+int kvm_load_realmode_segment(struct kvm_vcpu *vcpu, u16 selector, int seg)
+{
+	struct kvm_segment segvar = {
+		.base = selector << 4,
+		.limit = 0xffff,
+		.selector = selector,
+		.type = 3,
+		.present = 1,
+		.dpl = 3,
+		.db = 0,
+		.s = 1,
+		.l = 0,
+		.g = 0,
+		.avl = 0,
+		.unusable = 0,
+	};
+	kvm_x86_ops->set_segment(vcpu, &segvar, seg);
+	return 0;
+}
+
 int kvm_load_segment_descriptor(struct kvm_vcpu *vcpu, u16 selector,
 				int type_bits, int seg)
 {
 	struct kvm_segment kvm_seg;
 
+	if (!(vcpu->arch.cr0 & X86_CR0_PE))
+		return kvm_load_realmode_segment(vcpu, selector, seg);
 	if (load_segment_descriptor_to_kvm_desct(vcpu, selector, &kvm_seg))
 		return 1;
 	kvm_seg.type |= type_bits;