KVM: x86: advertise KVM_CAP_X86_SMM

... and we're done. :)

Because SMBASE is usually relocated above 1M on modern chipsets, and
SMM handlers might indeed rely on 4G segment limits, we only expose it
if KVM is able to run the guest in big real mode.  This includes any
of VMX+emulate_invalid_guest_state, VMX+unrestricted_guest, or SVM.

Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -2900,6 +2900,17 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 #endif
 		r = 1;
 		break;
+	case KVM_CAP_X86_SMM:
+		/* SMBASE is usually relocated above 1M on modern chipsets,
+		 * and SMM handlers might indeed rely on 4G segment limits,
+		 * so do not report SMM to be available if real mode is
+		 * emulated via vm86 mode.  Still, do not go to great lengths
+		 * to avoid userspace's usage of the feature, because it is a
+		 * fringe case that is not enabled except via specific settings
+		 * of the module parameters.
+		 */
+		r = kvm_x86_ops->cpu_has_high_real_mode_segbase();
+		break;
 	case KVM_CAP_COALESCED_MMIO:
 		r = KVM_COALESCED_MMIO_PAGE_OFFSET;
 		break;
@@ -4299,6 +4310,10 @@ static void kvm_init_msr_list(void)
 
 	for (i = j = 0; i < ARRAY_SIZE(emulated_msrs); i++) {
 		switch (emulated_msrs[i]) {
+		case MSR_IA32_SMBASE:
+			if (!kvm_x86_ops->cpu_has_high_real_mode_segbase())
+				continue;
+			break;
 		default:
 			break;
 		}